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/pkg/db/queries/runtime.sql

@@ -51,3 +51,14 @@ SET status = 'offline', updated_at = now()
 WHERE status = 'online'
   AND last_seen_at < now() - make_interval(secs => @stale_seconds::double precision)
 RETURNING id, workspace_id;
+
+-- name: FailTasksForOfflineRuntimes :many
+-- Marks dispatched/running tasks as failed when their runtime is offline.
+-- This cleans up orphaned tasks after a daemon crash or network partition.
+UPDATE agent_task_queue
+SET status = 'failed', completed_at = now(), error = 'runtime went offline'
+WHERE status IN ('dispatched', 'running')
+  AND runtime_id IN (
+    SELECT id FROM agent_runtime WHERE status = 'offline'
+  )
+RETURNING id, agent_id, issue_id;