refactor(runtime): consolidate hook orchestration and cache executors

[dgageot]

Summary

Continues the hooks-extraction refactoring: pulls more orchestration out of pkg/runtime, makes the pkg/hooks mechanism more powerful, and significantly simplifies the runtime package.

Net effect across pkg/runtime: −189 lines, all hook code colocated in a single hooks.go, no behaviour change. go test -race and golangci-lint both clean.

Changes

pkg/hooks — more ergonomic for builtin authors and embedders

• Executor.Dispatch now auto-fills Input.Cwd from the executor's working directory when callers leave it empty. Eliminates the per-callsite Cwd: r.workingDir plumbing across the runtime.
• New hooks.NewAdditionalContextOutput(event, content) helper for BuiltinFunc implementations that just want to contribute additional context. Returns nil for empty content.

pkg/hooks/builtins — owns the agent-flag → builtin wiring

• New AgentDefaults struct + ApplyAgentDefaults(cfg, defaults) *hooks.Config. Encodes the rules:
  • AddDate → turn_start (recompute every turn)
  • AddPromptFiles → turn_start (file may be edited mid-session)
  • AddEnvironmentInfo → session_start (cwd / OS / arch are session-stable)
• Returns nil when the resulting config is empty so the runtime can skip executor construction. Covered by three new tests.
• Builtins now use NewAdditionalContextOutput, dropping duplicated Output literal construction.

pkg/runtime — simpler and faster

• New pkg/runtime/hooks.go holds all hook orchestration:
  • hooksExec(a) replaces getHooksExecutor and caches the per-agent *hooks.Executor (including the nil "no hooks configured" sentinel) under an RWMutex. Per-turn dispatches no longer pay the matcher-compilation cost repeatedly.
  • dispatchHook(ctx, agent, event, input, events) is the single shared dispatcher: resolves the cached executor, short-circuits on no-hook, dispatches, logs errors, and emits Warning events for SystemMessage.
• The nine executeXHooks methods shrink from 10–25 lines each to a handful of lines, all delegating to dispatchHook.
• tool_dispatch.go: runTool no longer fetches the executor; pre/post-tool hooks now go through dispatchHook too. newHooksInput is a package function and no longer sets Cwd.

Diffstat

```
pkg/config/latest/hooks_yaml_test.go | 4 +-
pkg/hooks/builtins/builtins.go | 65 +++++--
pkg/hooks/builtins/builtins_test.go | 54 ++++++
pkg/hooks/executor.go | 6 +-
pkg/hooks/types.go | 16 ++
pkg/runtime/hooks.go | 230 +++++++++++++++++++++++++
pkg/runtime/hooks_wiring_test.go | 20 ++-
pkg/runtime/runtime.go | 317 +----------------------------------
pkg/runtime/tool_dispatch.go | 71 ++++----
9 files changed, 412 insertions(+), 371 deletions(-)
```

Validation

• `go build ./...` ✅
• `go vet ./...` ✅
• `go test ./...` ✅
• `go test -race ./pkg/runtime/... ./pkg/hooks/...` ✅
• `golangci-lint run ./...` → 0 issues

Assisted-By: docker-agent