recoverInterruptedMergingTasks searched for landed commits by grepping
commit subjects for the task ID. Users with includeTaskIdInCommit=false
have commit subjects like `feat: ...` (no task ID), so if the merger
crashed after committing but before storing mergeDetails, recovery would
silently fail to find the commit and incorrectly retry the merge.
Three layered defenses:
1. Emit a Fusion-Task-Id: <id> trailer in every Fusion-managed merge
commit body. The 4 fallback commit invocations now include
`-m "Fusion-Task-Id: ..."`. After the AI agent commits, an
idempotent ensureTaskIdTrailerOnHead() amends the trailer in via
`git interpret-trailers` (no-op if already present).
2. findLandedTaskCommit now tries three sources in order:
a. task.mergeDetails.commitSha (if reachable from HEAD)
b. Fusion-Task-Id trailer grep (anchored regex)
c. Subject grep (legacy commits)
3. Trailer grep uses an anchored regex `^Fusion-Task-Id: <id>$` so
it doesn't false-match task IDs appearing as substrings.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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