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/run-audit.ts

@@ -41,7 +41,7 @@
  * This ensures manual/non-run paths are unaffected by audit instrumentation.
  */
 
-import type { TaskStore, RunAuditEventInput, RunAuditDomain } from "@fusion/core";
+import type { TaskStore, RunAuditEventInput } from "@fusion/core";
 
 /** Structured context for a run correlation ID. */
 export interface EngineRunContext {