fix(FN-2117): log swallowed engine runtime errors

- Add structured warning logs in silent catch paths for IPC worker shutdown, plugin unregistration cleanup, child runtime metrics polling, and merger build rollback reset
- Improve merger rollback warning context to make build-verification reset and retry failures easier to diagnose
- Add regression tests covering each new warning path to ensure errors are surfaced without changing existing control flow
- Add StepSessionExecutor test coverage for cherry-pick abort logging when conflict cleanup fails

packages/engine/src/ipc/ipc-worker.test.ts

@@ -15,6 +15,7 @@
  */
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import { PING, PONG, OK, ERROR, TASK_CREATED, ERROR_EVENT } from "./ipc-protocol.js";
+import { ipcLog } from "../logger.js";
 
 // ── Mock logger to suppress console output ──────────────────────────────
 vi.mock("../logger.js", () => ({
@@ -408,6 +409,23 @@ describe("IpcWorker", () => {
       worker.removeAllListeners();
     });
 
+    it("logs warning when process.send throws during shutdown", () => {
+      const { worker, sendFn } = createWorker();
+      vi.mocked(ipcLog.warn).mockClear();
+
+      sendFn.mockImplementation(() => {
+        throw new Error("channel closed");
+      });
+
+      worker.shutdown();
+
+      expect(vi.mocked(ipcLog.warn)).toHaveBeenCalledWith(
+        expect.stringContaining("Failed to send SHUTDOWN message to parent: channel closed"),
+      );
+      expect(worker.isShuttingDown()).toBe(true);
+      worker.removeAllListeners();
+    });
+
     it('emits "shutdown" event on the IpcWorker instance', () => {
       const { worker } = createWorker();
       const handler = vi.fn();

packages/engine/src/ipc/ipc-worker.ts

@@ -278,8 +278,9 @@ export class IpcWorker extends EventEmitter {
     // Notify parent we're shutting down
     try {
       process.send?.({ type: "SHUTDOWN", id: generateCorrelationId(), payload: {} });
-    } catch {
-      // Ignore errors during shutdown
+    } catch (sendErr: unknown) {
+      const msg = sendErr instanceof Error ? sendErr.message : String(sendErr);
+      ipcLog.warn(`Failed to send SHUTDOWN message to parent: ${msg}`);
     }
   }