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/routine-scheduler.ts

@@ -169,7 +169,7 @@ export class RoutineScheduler {
     // Update next run time
     if (routine.cronExpression) {
       try {
-        const nextRun = CronExpressionParser.parse(routine.cronExpression).next();
+        const _nextRun = CronExpressionParser.parse(routine.cronExpression).next();
         // Note: We can't update nextRunAt directly as it's derived from trigger
         // The RoutineStore handles this internally
       } catch (err) {