feat: session.fork — branch a conversation, optionally onto another backend

[saucam]

What

Fork a session into a new, independent one that starts with a copy of the parent's conversation — and optionally continue the branch on a different backend in the same call. session.fork {sessionId, providerId: "codex"} = "branch this claude conversation and continue it on codex." The parent is untouched; both evolve independently. This is the purest expression of the meta-harness bet: one history, branched across backends to compare or diverge.

How

• CanonicalHistoryAccumulator.seed(turns) — replace history with a copy (fork priming; the fork never shares turn objects with the parent).
• Session: canonicalHistory getter + primeFromFork(history, transcript). Seeds the accumulator, replays the parent's transcript into the fork's scrollback for UI visibility — deliberately without tripping restoreScrollback's "prior scrollback ⇒ setHasQueried(true)", because a fork's backend is brand new and must run its first turn as a create, not a resume — and offers the history to the provider through an extracted #seedProviderFromHistory helper now shared with switchProvider (stateless backends replay history every turn; warm backends get a rendered transcript). Fresh id + fresh backing id, so the fork can never resume the parent's native state.
• SessionManager#fork: snapshots the parent's history + transcript, builds an independent Session (providerId = requested ?? parent's), primes it, audits. Fail-closed: foreign/unknown parent → not_found, unknown providerId → invalid_request, conductor → invalid_request.
• Protocol SessionForkMsg — wire-additive (new ClientMessage, no version bump); schema + exhaustiveness sample.

Composes with everything shipped: fork alone, fork + later set_provider, or fork-with-provider in one shot.

Tests

session-fork.test.ts: the primeFromFork primitive (history copied not shared, warm provider seeded, no-resume invariant) and the manager handler F1–F5 (history carried + (fork) name + new id; cross-provider fork with the fork's provider actually seeded; unknown provider rejected; cross-tenant not_found; unknown session not_found).

Verified live end-to-end on real subscriptions: told a claude session the codeword FORKTEST-42, forked it onto codex, and codex — a different backend — recalled FORKTEST-42. The conversation crossed.

bun run test (1275 pass), typecheck, lint green. Independent of #145 (rebased onto main).

🤖 Generated with Claude Code

[github-actions]

🤖 Gemini code review

This PR implements the session.fork feature, allowing a conversation to be branched into an independent session, optionally onto a different backend provider. While the implementation is largely clean and robust, there are critical issues around the persistence of the forked scrollback across daemon restarts and potential side-effects from shallow-copying nested structures in the conversation history.

Findings: 🔴 1 · 🟠 0 · 🟡 1 · 🟢 0

---

_{Tokens spent · ⬆️ Input: 8,058 · ⬇️ Output: 628 · Σ Total: 18,675
Total may be higher due to thinking token counts.}

[codecov]

Codecov Report

❌ Patch coverage is 97.39130% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.07%. Comparing base (d88adf5) to head (7034d7f).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/daemon/session.ts	91.17%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   82.82%   83.07%   +0.24%     
==========================================
  Files         104      104              
  Lines       17734    17823      +89     
==========================================
+ Hits        14688    14806     +118     
+ Misses       3046     3017      -29     

Flag	Coverage Δ
daemon	83.07% <97.39%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/protocol/src/schemas.ts	100.00% <100.00%> (ø)
packages/protocol/src/types.ts	100.00% <ø> (ø)
src/daemon/providers/canonical.ts	100.00% <100.00%> (ø)
src/daemon/session-manager.ts	68.43% <100.00%> (+2.94%)	⬆️
src/daemon/session.ts	95.34% <91.17%> (-0.10%)	⬇️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.