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/internal/handler/daemon.go

@@ -325,6 +325,18 @@ func (h *Handler) CompleteTask(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, taskToResponse(*task))
 }
 
+// GetTaskStatus returns the current status of a task.
+// Used by the daemon to check whether a task was cancelled mid-execution.
+func (h *Handler) GetTaskStatus(w http.ResponseWriter, r *http.Request) {
+	taskID := chi.URLParam(r, "taskId")
+	task, err := h.Queries.GetAgentTask(r.Context(), parseUUID(taskID))
+	if err != nil {
+		writeError(w, http.StatusNotFound, "task not found")
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]string{"status": task.Status})
+}
+
 // FailTask marks a running task as failed.
 type TaskFailRequest struct {
 	Error string `json:"error"`

server/internal/handler/issue.go

@@ -389,9 +389,11 @@ func (h *Handler) shouldEnqueueOnComment(ctx context.Context, issue db.Issue) bo
 	if !h.isAgentTriggerEnabled(ctx, issue, "on_comment") {
 		return false
 	}
-	// Don't enqueue if there's already an active task for this issue.
-	hasActive, err := h.Queries.HasActiveTaskForIssue(ctx, issue.ID)
-	if err != nil || hasActive {
+	// 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 for rapid-fire comments).
+	hasPending, err := h.Queries.HasPendingTaskForIssue(ctx, issue.ID)
+	if err != nil || hasPending {
 		return false
 	}
 	return true