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/skill-resolver.ts

@@ -385,7 +385,7 @@ export function createSkillsOverrideFromSelection(
 
     // Log diagnostics if any
     if (newDiagnostics.length > 0) {
-      const purpose = sessionPurpose ? `[${sessionPurpose}]` : "skills";
+      const _purpose = sessionPurpose ? `[${sessionPurpose}]` : "skills";
       for (const diag of newDiagnostics) {
         console.error(`[pi] [skills] ${diag.type}: ${diag.message}`);
       }