Plan: Story 919

ACs → implementation locations

AC 1 (MergeFailure + Unblock → Merge): server/src/pipeline_state/transition.rs:295 — change Ok(Coding) to Ok(Merge { feature_branch, commits_ahead }). Also requires adding feature_branch: BranchName and commits_ahead: NonZeroU32 to Stage::MergeFailure in server/src/pipeline_state/types.rs:113, and carrying those fields through the Merge → MergeFailure transition at transition.rs:200.
AC 2 (Coding (blocked) + Unblock → Coding unchanged): server/src/pipeline_state/transition.rs:289 — (Blocked { .. }, Unblock) => Ok(Coding) already correct; no change needed.
AC 3 (regression test): server/src/pipeline_state/tests.rs — add merge_failure_unblock_returns_to_merge CRDT-based test.

Add feature_branch and commits_ahead to MergeFailure (rather than use synthetic defaults in the transition): allows the exact merge state to be restored on Unblock. Rejected: synthetic values at transition time (would require story_id not available in the transition function, or use a placeholder that loses the real branch name).
Update all Stage::MergeFailure { reason } construction sites to include the new fields with synthetic defaults where real values are unavailable (CRDT read, from_dir, migration).

Not started — fresh session.

Add feature_branch: BranchName and commits_ahead: NonZeroU32 to Stage::MergeFailure in types.rs
Update types.rs from_dir construction of MergeFailure
Update transition.rs: carry fields in Merge → MergeFailure, self-loop, fix MergeFailure + Unblock → Merge
Update crdt_state/read.rs MergeFailure construction
Update crdt_state/write/migrations.rs MergeFailure construction
Update tests.rs: all Stage::MergeFailure { .. } constructions, rename+fix unblock test
Add CRDT-based regression test (AC3)
Run run_check, fix any clippy/fmt issues
Run run_tests, commit