fix(cron): skip relay only for explicit delivery config, not legacy payload

Fixes #15692

The previous fix was too broad — it removed the relay for ALL isolated jobs.
This broke backwards compatibility for jobs without explicit delivery config.

The correct behavior is:
- If job.delivery exists → isolated runner handles it via runSubagentAnnounceFlow
- If only legacy payload.deliver fields → relay to main if requested (original behavior)

This addresses Greptile's review feedback about runIsolatedAgentJob being an
injected dependency that might not call runSubagentAnnounceFlow.

Uses resolveCronDeliveryPlan().source to distinguish between explicit delivery
config and legacy payload-only jobs.

src/cron/service.delivery-plan.test.ts

@@ -89,4 +89,47 @@ describe("CronService delivery plan consistency", () => {
     cron.stop();
     await store.cleanup();
   });
+
+  it("does not enqueue duplicate relay when isolated run marks delivery handled", async () => {
+    const store = await makeStorePath();
+    const enqueueSystemEvent = vi.fn();
+    const requestHeartbeatNow = vi.fn();
+    const runIsolatedAgentJob = vi.fn(async () => ({
+      status: "ok" as const,
+      summary: "done",
+      delivered: true,
+    }));
+    const cron = new CronService({
+      cronEnabled: true,
+      storePath: store.storePath,
+      log: noopLogger,
+      enqueueSystemEvent,
+      requestHeartbeatNow,
+      runIsolatedAgentJob,
+    });
+    await cron.start();
+    const job = await cron.add({
+      name: "announce-delivered",
+      schedule: { kind: "every", everyMs: 60_000, anchorMs: Date.now() },
+      sessionTarget: "isolated",
+      wakeMode: "now",
+      payload: {
+        kind: "agentTurn",
+        message: "hello",
+      },
+      delivery: { channel: "telegram", to: "123" } as unknown as {
+        mode: "none" | "announce";
+        channel?: string;
+        to?: string;
+      },
+    });
+
+    const result = await cron.run(job.id, "force");
+    expect(result).toEqual({ ok: true, ran: true });
+    expect(enqueueSystemEvent).not.toHaveBeenCalled();
+    expect(requestHeartbeatNow).not.toHaveBeenCalled();
+
+    cron.stop();
+    await store.cleanup();
+  });
 });