refactor(heartbeat): centralize [Heartbeat] prefix as a shared constant

P2-36: Multiple subsystems were string-coupled via the magic
'[Heartbeat]' prefix. Introduce src/main/orchestration/heartbeat-task.ts
as the single source of truth and replace scattered literals in
heartbeat dispatch, automation actions, and run completion.

P3-52: Collapse 5-deep ternary in automation-actions' wake-trigger
selection into lookup helpers (deriveHeartbeatWakeTrigger,
deriveEscalationWakeTrigger).

src/main/orchestration/heartbeat-task.ts

@@ -0,0 +1,15 @@
+/**
+ * Heartbeat task title conventions.
+ *
+ * Heartbeat tasks are synthesized by the orchestrator when an agent wakes with
+ * no actionable work; they carry the `[Heartbeat]` title prefix so the rest of
+ * the system can detect them without a dedicated `kind` column.
+ *
+ * Centralizing the prefix here prevents drift between the writer (heartbeat
+ * dispatch) and readers (automation, escalation, route filters).
+ */
+export const HEARTBEAT_TASK_TITLE_PREFIX = "[Heartbeat]";
+
+export function isHeartbeatTaskTitle(title: string | null | undefined): boolean {
+  return !!title && title.startsWith(HEARTBEAT_TASK_TITLE_PREFIX);
+}