fix: handle Unix kill errors in orphaned process cleanup

Replace execAsync kill command with individual process.kill calls wrapped
in try/catch to gracefully handle processes that have already exited.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/worker-service.ts

@@ -451,7 +451,13 @@ export class WorkerService {
         execSync(`taskkill /PID ${pid} /T /F`, { timeout: 60000, stdio: 'ignore' });
       }
     } else {
-      await execAsync(`kill ${pids.join(' ')}`);
+      for (const pid of pids) {
+        try {
+          process.kill(pid, 'SIGKILL');
+        } catch {
+          // Process already exited - that's fine
+        }
+      }
     }
 
     logger.info('SYSTEM', 'Orphaned processes cleaned up', { count: pids.length });