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/runtimes/child-process-runtime.test.ts

@@ -6,6 +6,7 @@ import type {
   RuntimeMetrics,
   RuntimeStatus,
 } from "../project-runtime.js";
+import { runtimeLog } from "../logger.js";
 import {
   START_RUNTIME,
   STOP_RUNTIME,
@@ -673,6 +674,36 @@ describe("ChildProcessRuntime", () => {
       });
     });
 
+    it("logs warning when GET_METRICS IPC query fails", async () => {
+      const warnSpy = vi.spyOn(runtimeLog, "warn").mockImplementation(() => {});
+
+      queueChild({
+        sendCallbackErrors: {
+          [GET_METRICS]: new Error("metrics unavailable"),
+        },
+      });
+      await runtime.start();
+
+      runtimeAny.lastMetrics = {
+        inFlightTasks: 1,
+        activeAgents: 0,
+        lastActivityAt: "2026-04-08T04:00:00.000Z",
+      };
+
+      const metrics = runtime.getMetrics();
+      expect(metrics.inFlightTasks).toBe(1);
+      expect(metrics.activeAgents).toBe(0);
+
+      await vi.waitFor(() => {
+        expect(warnSpy).toHaveBeenCalledWith(
+          expect.stringContaining("GET_METRICS IPC query failed, using cached value"),
+        );
+      });
+      expect(warnSpy).toHaveBeenCalledWith(expect.stringContaining("metrics unavailable"));
+
+      warnSpy.mockRestore();
+    });
+
     it("getTaskStore() always throws not accessible error", () => {
       expect(() => runtime.getTaskStore()).toThrow("not accessible in ChildProcessRuntime");
     });

packages/engine/src/runtimes/child-process-runtime.ts

@@ -450,8 +450,9 @@ export class ChildProcessRuntime
         .then((metrics: unknown) => {
           this.lastMetrics = metrics as RuntimeMetrics;
         })
-        .catch(() => {
-          // Ignore errors, use cached value
+        .catch((err: unknown) => {
+          const msg = err instanceof Error ? err.message : String(err);
+          runtimeLog.warn(`GET_METRICS IPC query failed, using cached value: ${msg}`);
         });
     }