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/merger.ts

@@ -1298,6 +1298,14 @@ export async function aiMergeTask(
   }
 
   // 5. Execute merge with retry logic
+  // Cross-process safety net: abort if another task is already mid-merge.
+  // The engine's drainMergeQueue also checks, but this catches direct callers.
+  const activeMerge = store.getActiveMergingTask(taskId);
+  if (activeMerge) {
+    throw new Error(
+      `Cannot merge ${taskId}: task ${activeMerge} is already merging (cross-process conflict)`,
+    );
+  }
   await store.updateTask(taskId, { status: "merging" });
 
   // Normalize explicit verification commands from settings