fix(cron): share isolated announce flow + harden cron scheduling/delivery (#11641)

* fix(cron): comprehensive cron scheduling and delivery fixes

- Fix delivery target resolution for isolated agent cron jobs
- Improve schedule parsing and validation
- Add job retry logic and error handling
- Enhance cron ops with better state management
- Add timer improvements for more reliable cron execution
- Add cron event type to protocol schema
- Support cron events in heartbeat runner (skip empty-heartbeat check,
  use dedicated CRON_EVENT_PROMPT for relay)

* fix: remove cron debug test and add changelog/docs notes (#11641) (thanks @tyler6204)

src/agents/subagent-announce.format.test.ts

@@ -94,7 +94,7 @@ describe("subagent announce formatting", () => {
     };
     const msg = call?.params?.message as string;
     expect(call?.params?.sessionKey).toBe("agent:main:main");
-    expect(msg).toContain("background task");
+    expect(msg).toContain("subagent task");
     expect(msg).toContain("failed");
     expect(msg).toContain("boom");
     expect(msg).toContain("Findings:");
@@ -155,7 +155,7 @@ describe("subagent announce formatting", () => {
     expect(didAnnounce).toBe(true);
     expect(embeddedRunMock.queueEmbeddedPiMessage).toHaveBeenCalledWith(
       "session-123",
-      expect.stringContaining("background task"),
+      expect.stringContaining("subagent task"),
     );
     expect(agentSpy).not.toHaveBeenCalled();
   });