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/openclaw-tools.subagents.sessions-spawn-resolves-main-announce-target-from.test.ts

@@ -153,7 +153,7 @@ describe("openclaw-tools: subagents", () => {
     // Second call: main agent trigger (not "Sub-agent announce step." anymore)
     const second = agentCalls[1]?.params as { sessionKey?: string; message?: string } | undefined;
     expect(second?.sessionKey).toBe("main");
-    expect(second?.message).toContain("background task");
+    expect(second?.message).toContain("subagent task");
 
     // No direct send to external channel (main agent handles delivery)
     const sendCalls = calls.filter((c) => c.method === "send");