fusion

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gsxdsm 7763ba5fab FN-6278: preflight reusable merge worktree cwd

Stabilize reusable merge worktree cwd resolution before merge-runner git spawns.

- Add a reusable merge integration root preflight that detects missing, empty, or de-registered task worktrees before using them as cwd.
- Reacquire/repair unusable reuse-task-worktree roots before merge execution and reject unusable roots during handoff.
- Cover vanished and unregistered worktree recovery paths plus focused preflight unit cases.
- Document the FN-6278 preflight behavior alongside transient merge recovery and merge integration worktree contracts.

Files changed:
 docs/architecture.md                               |   4 +-
 .../__tests__/merger-integration-worktree.test.ts  |  82 ++++++++
 .../merge-runner-spawn-enoent-prevention.test.ts   | 233 +++++++++++++++++++++
 packages/engine/src/merger-integration-worktree.ts |  71 +++++++
 packages/engine/src/merger.ts                      |  19 +-
 5 files changed, 396 insertions(+), 13 deletions(-)

Fusion-Task-Id: FN-6278

Fusion-Task-Lineage: ceecf70f-9e2d-4ffe-8c6f-a9a5b673d72d

2026-06-11 20:27:29 -07:00