diff --git a/docs/README.md b/docs/README.md
index da077b25..9a4e5d43 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -13,6 +13,7 @@ Start with [architecture.md](architecture.md) for the current backend model and
 | Doc | What it covers |
 |-----|----------------|
 | [architecture.md](architecture.md) | Current backend model, package layout, status derivation, persistence/CDC, and load-bearing rules. |
+| [backend-code-structure.md](backend-code-structure.md) | Package ownership rules for the Go backend: domain, services, ports, adapters, storage, HTTP, CLI, and daemon wiring. |
 | [cli/README.md](cli/README.md) | CLI commands and daemon control surface. |
 | [status.md](status.md) | Current implementation shape, build/test command, and next integration work. |
 | [stack.md](stack.md) | Accepted library/runtime choices, pending stack decisions, and dependencies explicitly avoided for V1. |
diff --git a/docs/backend-code-structure.md b/docs/backend-code-structure.md
new file mode 100644
index 00000000..68d9e008
--- /dev/null
+++ b/docs/backend-code-structure.md
@@ -0,0 +1,382 @@
+# Backend Code Structure
+
+This document describes package ownership for the Go backend. It is about where
+code belongs. See [architecture.md](architecture.md) for lifecycle behavior,
+status derivation, persistence, CDC, and invariants.
+
+## Goal
+
+The backend is a local daemon that supervises coding-agent sessions. The code
+needs clear homes for product workflows, protocol surfaces, persistence, and
+replaceable external systems without turning any single package into a catch-all.
+
+The current structure is a layered hybrid:
+
+- `domain` holds shared product vocabulary and durable fact records.
+- `service/*` owns controller-facing product use cases and read models.
+- `session_manager` owns internal session mutations and resource orchestration.
+- `lifecycle` owns the durable session fact reducer.
+- `ports` defines narrow capability interfaces consumed by core code.
+- `adapters/*` implements those capabilities with real external systems.
+- `storage/sqlite` and `cdc` own persistence and change delivery.
+- `httpd` and `cli` own protocol concerns.
+- `daemon` wires the production graph together.
+
+## Package Roles
+
+### `internal/domain`
+
+`domain` is AO's shared product language. Keep it stable and free of
+infrastructure imports.
+
+Belongs here:
+
+- shared IDs such as `ProjectID`, `SessionID`, and `IssueID`;
+- shared enums and status vocabulary;
+- durable fact records that multiple packages must agree on;
+- PR, tracker, project, and session vocabulary that is not transport-specific.
+
+Does not belong here:
+
+- HTTP request/response DTOs;
+- CLI output shapes;
+- OpenAPI wrapper/envelope types;
+- sqlc generated rows;
+- GitHub, Zellij, Claude, Codex, or OpenCode payloads;
+- one-resource controller helper types.
+
+Rule of thumb: if AO would still use the concept after replacing HTTP, the CLI,
+SQLite, GitHub, Zellij, and every agent adapter, and more than one package needs
+the exact vocabulary, it may belong in `domain`.
+
+### `internal/service/*`
+
+`service` packages are the controller-facing application boundary.
+
+Current examples:
+
+```txt
+internal/service/project
+internal/service/session
+internal/service/pr
+```
+
+Belongs here:
+
+- resource use cases called by HTTP controllers and CLI-backed API flows;
+- resource read models and command/result types;
+- display-model assembly, such as session status derived from session and PR
+  facts;
+- resource-specific validation and user-facing errors;
+- small store interfaces consumed by the service.
+
+Does not belong here:
+
+- low-level runtime/workspace/agent process control;
+- raw sqlc generated rows as public service results;
+- HTTP routing, path parsing, status-code decisions, or OpenAPI generation;
+- concrete external adapter details.
+
+For example, project API concepts live in `internal/service/project`, not in
+`domain` and not in a top-level `internal/project` package.
+
+### `internal/session_manager`
+
+`session_manager` owns internal session commands: spawn, restore, kill, cleanup,
+and send-related orchestration over runtime, workspace, agent, storage,
+messenger, and lifecycle dependencies.
+
+Belongs here:
+
+- multi-step session mutations;
+- rollback/cleanup sequencing when spawn partially succeeds;
+- resource teardown safety;
+- internal errors such as not found, terminated, or not restorable.
+
+Does not belong here:
+
+- HTTP request decoding;
+- CLI formatting;
+- controller-facing list/get read-model assembly;
+- terminal WebSocket framing.
+
+The split is intentional: `service/session` is the product/API boundary;
+`session_manager` is the internal command engine.
+
+### `internal/lifecycle`
+
+`lifecycle` is the canonical write path for durable session lifecycle facts. It
+reduces runtime observations, activity signals, spawn completion, termination,
+and PR observations into small persisted facts.
+
+Belongs here:
+
+- updates to lifecycle-owned session facts;
+- guardrails around runtime/activity observations;
+- lifecycle-triggered agent nudges for actionable PR facts.
+
+Does not belong here:
+
+- display status persistence;
+- HTTP/CLI DTOs;
+- direct adapter implementation details;
+- PR row persistence.
+
+The UI status is derived at read time by service code. Do not store display
+status in lifecycle or SQLite.
+
+### `internal/ports`
+
+`ports` contains narrow capability interfaces and shared adapter-facing structs.
+It connects core code to replaceable systems.
+
+Current capability examples:
+
+- `Runtime`
+- `Workspace`
+- `Agent`
+- `AgentResolver`
+- `AgentMessenger`
+- `PRWriter`
+
+Belongs here:
+
+- interfaces consumed by core packages and implemented by adapters;
+- capability structs such as `RuntimeConfig`, `WorkspaceConfig`, and
+  `SpawnConfig`;
+- vocabulary needed at the boundary between core orchestration and adapters.
+
+Does not belong here:
+
+- resource read models like project/session API responses;
+- HTTP request/response DTOs;
+- sqlc rows;
+- concrete adapter options;
+- one-off interfaces that only a single package needs internally.
+
+Keep `ports` capability-oriented. It should not become the dumping ground for
+every manager, DTO, and resource contract.
+
+### `internal/adapters/*`
+
+Adapters are concrete implementations of external systems.
+
+Current examples:
+
+```txt
+internal/adapters/agent/claudecode
+internal/adapters/agent/codex
+internal/adapters/agent/opencode
+internal/adapters/runtime/zellij
+internal/adapters/workspace/gitworktree
+internal/adapters/scm/github
+internal/adapters/tracker/github
+```
+
+Adapters should be leaves in the import graph. They translate external behavior
+into AO ports and domain concepts; they should not own product workflows.
+
+Good:
+
+```txt
+session_manager -> ports.Runtime
+adapters/runtime/zellij -> ports + domain
+adapters/workspace/gitworktree -> ports + domain
+daemon -> adapters + services + storage
+```
+
+Avoid:
+
+```txt
+domain -> adapters
+service/session -> adapters/runtime/zellij
+httpd/controllers -> storage/sqlite/store
+adapters/* -> httpd
+```
+
+### `internal/storage/sqlite`
+
+`storage/sqlite` owns SQLite setup, migrations, sqlc generated code, and store
+implementations.
+
+Belongs here:
+
+- connection setup and PRAGMAs;
+- goose migrations;
+- sqlc queries and generated code;
+- table-specific store methods;
+- transactions and CDC-triggered persistence behavior.
+
+Does not belong here:
+
+- HTTP response types;
+- CLI output formatting;
+- product display status rules;
+- external adapter logic.
+
+Generated sqlc types should stay behind store methods. Services and lifecycle
+code should work with domain records or service read models, not generated rows.
+
+### `internal/cdc`
+
+`cdc` owns `change_log` polling and event broadcasting. SQLite triggers append
+durable events to `change_log`; the poller tails that table and fans events out
+to subscribers.
+
+Belongs here:
+
+- event type definitions for the CDC stream;
+- poller and broadcaster logic;
+- subscriber fan-out behavior.
+
+Does not belong here:
+
+- terminal byte streams;
+- product workflow decisions;
+- database schema ownership.
+
+### `internal/terminal`
+
+`terminal` owns the terminal session protocol and PTY/session management used by
+the HTTP mux.
+
+Belongs here:
+
+- terminal session lifecycle;
+- input/output framing independent of HTTP;
+- PTY-backed session handling;
+- ring buffers and terminal protocol tests.
+
+`httpd` adapts WebSocket connections to terminal interfaces; `terminal` should
+not import `httpd`.
+
+### `internal/httpd`
+
+`httpd` is the HTTP protocol adapter.
+
+Belongs here:
+
+- routing and middleware;
+- HTTP request decoding and response encoding;
+- path/query parameter handling;
+- status-code mapping;
+- API error envelopes;
+- OpenAPI generation and serving;
+- WebSocket upgrade handling for terminal mux.
+
+Controllers call service managers and translate service results/errors into HTTP
+responses. Controllers should not reach directly into concrete adapters or the
+SQLite store.
+
+HTTP-only request/response wrappers belong in `httpd` or
+`httpd/controllers`. Application read models shared by controller and CLI flows
+belong in the owning `service/*` package.
+
+### `internal/cli`
+
+`cli` owns the user-facing `ao` command. It should stay thin:
+
+- discover the local daemon;
+- call the daemon's loopback HTTP API;
+- format command output;
+- start/stop/status/doctor process control.
+
+The CLI should not duplicate daemon business logic. If a command needs product
+behavior, put the behavior in the daemon service/API path and have the CLI call
+that path.
+
+### `internal/daemon`
+
+`daemon` is the production composition root. It wires config, logging, SQLite,
+CDC, lifecycle, reaper, runtime, terminal manager, services, HTTP, and shutdown.
+
+Belongs here:
+
+- production dependency construction;
+- adapter registration;
+- startup/shutdown sequencing;
+- cross-component wiring.
+
+Does not belong here:
+
+- business logic that should be testable in service, lifecycle, or manager
+  packages;
+- adapter implementation details.
+
+## Interface Placement
+
+Prefer interfaces near their consumers, except for shared capabilities.
+
+- If only one package consumes an abstraction, define the smallest interface in
+  that package.
+- If multiple core packages consume a replaceable capability, define it in
+  `ports`.
+- If HTTP controllers need a resource service, use the owning `service/*`
+  manager interface.
+- Return concrete types from constructors unless callers genuinely need an
+  interface.
+
+## Current Tree
+
+The current main-line shape is:
+
+```txt
+backend/
+  cmd/ao/                       # CLI entrypoint
+  main.go                       # daemon entrypoint compatibility
+  sqlc.yaml
+
+  internal/domain/              # shared product vocabulary and durable facts
+  internal/ports/               # capability interfaces
+  internal/service/
+    project/                    # project API/use-case boundary
+    session/                    # session API/use-case boundary
+    pr/                         # PR observation/action service
+  internal/session_manager/     # internal session command engine
+  internal/lifecycle/           # durable lifecycle fact reducer
+  internal/observe/reaper/      # runtime observation loop
+  internal/storage/sqlite/      # DB, migrations, queries, generated sqlc, stores
+  internal/cdc/                 # change_log poller and broadcaster
+  internal/terminal/            # terminal session protocol and PTY handling
+  internal/httpd/               # HTTP API, controllers, OpenAPI, terminal mux
+  internal/cli/                 # user-facing ao command
+  internal/daemon/              # production wiring and shutdown
+  internal/config/              # daemon env/default config
+  internal/adapters/            # concrete agent/runtime/workspace/SCM/tracker adapters
+```
+
+## Adding New Code
+
+Use these defaults:
+
+- New HTTP route: add controller/API code in `httpd`, call a `service/*`
+  package, and update OpenAPI generation/spec tests.
+- New product resource: put shared IDs/vocabulary in `domain`, use cases and
+  read models in `service/<resource>`, storage in `storage/sqlite`, and external
+  system seams in `ports`.
+- New adapter: implement a `ports` interface under `adapters/<capability>/<impl>`
+  and wire it in `daemon`.
+- New persisted fact: add a migration, sqlc query, store method, domain record or
+  event vocabulary, and CDC behavior when the UI/API must observe it.
+- New CLI command: keep command parsing/formatting in `cli`; call the daemon API
+  rather than reimplementing daemon behavior.
+
+## Project Routes Example
+
+Project-owned concepts live in `internal/service/project`:
+
+- project read models;
+- project add/remove command types;
+- project validation and user-facing errors;
+- the `Manager` contract consumed by HTTP controllers.
+
+`internal/httpd/controllers` remains responsible for:
+
+- route registration;
+- JSON decoding/encoding;
+- HTTP status codes and error envelopes;
+- mapping service errors to responses.
+
+When a type is ambiguous, ask whether it is a product use-case/read model or an
+HTTP wire wrapper. Product use-case/read models belong in `service/project`;
+HTTP wire wrappers belong in `httpd`.