refactor(runtime): extract session branching into session-branch.ts

[UncertaintyDeterminesYou4ndMe]

Part of #1084 — the session-manager.ts slice from the runtime/other lane (claimed in #1084 (comment)).

What moved

Behavior-neutral pure move of the branch-session subsystem (~250 lines) out of session-manager.ts into a new leaf module packages/runtime/src/session-branch.ts:

• createBranchSession and cloneBranchRuntimeLedger, behind a minimal structural SessionBranchDeps seam (store is an inline structural subset of the manager-local SessionStore, following the RuntimeLedgerRepairDeps precedent, so the leaf never imports back into session-manager.ts).
• The turn-boundary copy helpers copyMessagesThroughTurnBoundary / copyMessagesBeforeTurn (exported; branchFromTurn / branchBeforeTurn stay behind as thin public methods).
• The clone helpers cloneRuntimeEventForBranch / cloneRunHeaderForBranchCreate (module-private).

One deliberate seam adjustment: createBranchSession now returns the child SessionHeader; SessionManager wraps it with headerToSummary at the call sites, keeping the summary projection with its owner and the import direction one-way.

Dedup

headerLineage in session-manager.ts was a verbatim private copy of the one in agent-run-recovery.ts. The recovery module's copy is now exported and imported; the duplicate is deleted.

Invariants (per #1084 / #363)

• No logic changes; comments moved verbatim with their code.
• @maka/runtime public exports unchanged (index.ts untouched).
• No reverse imports from the new leaf.
• Flat branch off main, single slice.

Verification

• npm --workspace @maka/runtime run build — clean.
• npm --workspace @maka/runtime run test — 2269 tests, 2261 pass, 0 fail, 7 skipped, 1 cancelled. The cancelled one is the known pre-existing test(runtime): session-title deadline-abort test hangs under full-suite load on main #1222 (session-title deadline-abort, Node v22.17.0), reproduced identically before and after this change; all branchFromTurn/branchBeforeTurn tests pass.
• biome check clean on the three touched files.

[UncertaintyDeterminesYou4ndMe]

CI test caught a source-grep contract this move had to co-migrate: session-sticky-model-contract.test.ts asserted branchFromTurn … model: header.model in one session-manager.ts read, and the sticky-model capture now lives in session-branch.ts. Fixed in 24112ee by asserting each side of the seam where it lives — branchFromTurn delegates to createBranchSession (session-manager) and createBranchSession carries model: header.model (session-branch). Verified locally: desktop build + the contract test file pass; also scanned the repo for other source-reading tests touching the moved code (only session-startup-recovery-contract.test.ts reads session-manager.ts, and everything it asserts stayed put).

[UncertaintyDeterminesYou4ndMe]

Latest CI on this PR: test and e2e pass (the co-migrated contract fix landed). The remaining typecheck failure is not from this branch — it's the Biome format gate failing on main itself since #1254 (one unformatted file, packages/storage/src/__tests__/settings-store-usage.test.ts), inherited through the merge ref. Filed #1260 with the one-file format fix; once that lands, a re-run here should go green.

[Astro-Han]

I think main is clear on typecheck now, we can merge main or run biome linter locally to fix it.

[Astro-Han]

No code findings. The extraction preserves the existing branching behavior. Please rebase or rerun CI before merge because the current typecheck check is red on the old formatting baseline.

[Astro-Han]

Hi — this PR conflicts with current main and cannot be merged as-is.

I tested a rebase onto current main locally (in a throwaway worktree — your branch was not touched). It stops on these files:

• packages/runtime/src/session-manager.ts

These are real source conflicts, so they need your judgement rather than a mechanical rebase — please rebase onto current main and resolve them yourself, then push. Once the branch is conflict-free and CI is green on the new head, I will pick it up for review.

git fetch upstream && git rebase upstream/main
# resolve, then
git push --force-with-lease

Thanks for the contribution — happy to help if any conflict is unclear.

---

AI-assisted maintenance note, not a review. It does not count as the required human review under CONTRIBUTING.md §Review.

[UncertaintyDeterminesYou4ndMe]

Thanks for testing the rebase. Before resolving the conflict I re-checked whether the extraction still stands on current main — it doesn't, so I'm closing this instead of rebasing it.

What this PR moved into session-branch.ts (the ledger clone machinery, the copy-slice helpers, and createBranchSession) has since been extracted on main by a different, more general route:

• packages/runtime/src/conversation-copy.ts (introduced in feat(runtime-host): own session revision authority #1664, extended through fix(core): read AgentRun ledgers written by any version #1986/feat(runtime-host): preserve Agent Graph history across revisions #2039) now owns the slice/clone machinery (createConversationCopySlice, cloneConversationRuntimeLedger, …) with its own test suite — and serves branching, the newer revision sessions, and cross-Session copy from one place. Re-extracting a branch-only session-branch.ts on top of it would fight that factoring rather than complete it.
• What remains in session-manager.ts (branchFromTurn / branchBeforeTurn / createBranchSession, now alongside reviseBeforeTurn / createRevisionSession) are thin orchestrators over that module — the ~200-line extraction this PR carried no longer exists to extract.
• The two supporting changes are moot on main too: the headerLineage export was only needed by the extraction, and session-sticky-model-contract.test.ts (the co-migrated contract) has been deleted entirely.

So the goal of this refactor was reached on main via conversation-copy.ts, the same way #2476's goal was reached via run-command-core.ts. Closing as superseded.