feat(FN-1757): clear residual lint debt in engine package

- Remove unused imports across 25 files in engine package
- Remove unused variable declarations in ipc-worker.ts, child-process-runtime.ts, and mission-autopilot.ts
- Clean up unnecessary imports in agent-instructions.ts, agent-tools.ts, cron-runner.ts, executor.ts, and other modules
- Minor cleanup in notifier.ts, peer-exchange-service.ts, pi.ts, plugin-runner.ts, and other files
- Improves code quality and reduces potential confusion from unused code

packages/engine/src/plugin-runner.ts

@@ -502,7 +502,7 @@ export class PluginRunner {
         this.hookTimeoutMs,
         `Hook ${hookName} timed out`,
       );
-    } catch (err) {
+    } catch (_err) {
       // Error already logged by invokeHook
     }
   }