feat(tasks): add coalescing queue and task lifecycle guards

- Coalescing queue: use HasPendingTaskForIssue (queued/dispatched only)
  instead of HasActiveTaskForIssue so comments during a running task
  enqueue exactly one follow-up task that picks up all new comments.
- Stale task cleanup: runtime sweeper now fails orphaned tasks when
  their runtime goes offline (daemon crash/network partition).
- Cancel-aware daemon: handleTask checks task status after execution
  and discards results if the task was cancelled mid-run (e.g. reassign).
- Terminal issue guard: ClaimTaskForRuntime auto-cancels pending tasks
  for done/cancelled issues instead of executing them.
- Race condition safety net: unique partial index ensures at most one
  pending task per issue at the DB level.

server/migrations/022_task_lifecycle_guards.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS idx_one_pending_task_per_issue;