fix: handle process exit in waitForProcessesExit filter

The process.kill(pid, 0) call throws when a process has exited,
which crashed the filter callback. Wrapped in try/catch to
correctly return false for exited processes.

Fixes critical bug found in PR #458 review (Phase 1).

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

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

src/services/worker-service.ts

@@ -754,8 +754,12 @@ export class WorkerService {
 
     while (Date.now() - start < timeoutMs) {
       const stillAlive = pids.filter(pid => {
-        process.kill(pid, 0); // Signal 0 checks if process exists - throws if dead
-        return true;
+        try {
+          process.kill(pid, 0);
+          return true;
+        } catch {
+          return false;
+        }
       });
 
       if (stillAlive.length === 0) {