fix(engine): prevent two tasks showing "merging" status simultaneously

Root cause: if a merge crashed or the process restarted mid-merge, the
"merging" status was never cleared. On next startup the stale task kept
its "merging" status while the queue moved on to the next task, resulting
in two tasks appearing to merge at once.

Two fixes:
1. Add "merging"/"merging-pr" to BLOCKING_TASK_STATUSES so tasks with
   active merge status are not re-enqueued by the retry sweep.
2. Clear stale "merging" statuses during startup merge sweep — no merge
   is actually running at engine start, so any such status is a leftover.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core/src/task-merge.ts

@@ -4,6 +4,8 @@ const BLOCKING_TASK_STATUSES = new Set([
   "failed",
   "awaiting-inspection",
   "awaiting-user-review",
+  "merging",
+  "merging-pr",
 ]);
 
 const NON_TERMINAL_STEP_STATUSES = new Set([