Stop losing uncommitted dev edits during task merges.
- `acquireReuseHandoff` no longer throws MergeHandoffRefusedError("working-tree-dirty") on a dirty reused worktree (FN-5138). It autostashes via `git add -A` + `git stash create` + `git stash store`, emits a `merge:reuse-handoff-autostash` audit event with the stash SHA and a recover command, and lets the merge proceed.
- `stashUnrelatedRootDirChanges` no longer silently proceeds when stash creation fails on a dirty tree. It throws a new `AutostashCreationFailedError`; the merger catches it and surfaces a clear "your edits are intact" message before any destructive op runs.
- New failure reason `dirty-worktree-autostash-failed` distinguishes stash failure from the old refusal.
- Tests in `merger-integration-worktree`, `merger-cwd-fallback-removed`, and `reliability-interactions/{integration-worktree-state,merge-reuse-task-worktree,cwd-integration-fallback-removed}` updated to the new contract; the FN-5348 "no cwd fallback" invariant is preserved.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
FN-5351 adds structured telemetry for merge audit events and integration worktree state, including typed ref-advance tracking, terminal handoff fallback audit, and integration state probes with corresponding reliability backstop tests and documentation updates.
Fusion-Task-Id: FN-5351
