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/generated/agent.sql.go

@@ -358,6 +358,22 @@ func (q *Queries) HasActiveTaskForIssue(ctx context.Context, issueID pgtype.UUID
 	return has_active, err
 }
 
+const hasPendingTaskForIssue = `-- name: HasPendingTaskForIssue :one
+SELECT count(*) > 0 AS has_pending FROM agent_task_queue
+WHERE issue_id = $1 AND status IN ('queued', 'dispatched')
+`
+
+// Returns true if there is a queued or dispatched (but not yet running) task for the issue.
+// Used by the coalescing queue: allow enqueue when a task is running (so
+// the agent picks up new comments on the next cycle) but skip if a pending
+// task already exists (natural dedup).
+func (q *Queries) HasPendingTaskForIssue(ctx context.Context, issueID pgtype.UUID) (bool, error) {
+	row := q.db.QueryRow(ctx, hasPendingTaskForIssue, issueID)
+	var has_pending bool
+	err := row.Scan(&has_pending)
+	return has_pending, err
+}
+
 const listAgentTasks = `-- name: ListAgentTasks :many
 SELECT id, agent_id, issue_id, status, priority, dispatched_at, started_at, completed_at, result, error, created_at, context, runtime_id, session_id, work_dir FROM agent_task_queue
 WHERE agent_id = $1