fix(engine): add cross-process merge guard to prevent concurrent merges

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>

packages/engine/src/restart.integration.test.ts

@@ -145,6 +145,7 @@ function createMockStore(overrides: Record<string, any> = {}) {
       return makeTask("FN-NEW", "triage");
     }),
     deleteTask: vi.fn().mockResolvedValue(undefined),
+    getActiveMergingTask: vi.fn().mockReturnValue(undefined),
     _trigger(event: string, ...args: any[]) {
       for (const fn of listeners.get(event) || []) fn(...args);
     },