Multiple engine processes (dashboard + serve) share the same SQLite database
but each has its own in-memory merge queue. Without a cross-process check,
two processes can start merging different tasks simultaneously.
Added store.getActiveMergingTask() as a DB-level check before any merge
starts. The drainMergeQueue defers with pollIntervalMs delay, and both
aiMergeTask and processPullRequestMergeTask have safety-net checks.
Also moved stale merge status cleanup to run regardless of autoMerge setting.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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