fix: prevent zombie process accumulation via PID registry and signal propagation (Issue #737) (#806)

* Fix zombie process accumulation (Issue #737)

Problem: Claude haiku subprocesses spawned by the SDK weren't terminating
properly, causing zombie process accumulation (user reported 155 processes
consuming 51GB RAM).

Root causes:
1. SDK's SpawnedProcess interface hides subprocess PIDs
2. deleteSession() didn't verify subprocess exit
3. abort() was fire-and-forget with no confirmation
4. No mechanism to track or clean up orphaned processes

Solution:
- Add ProcessRegistry module to track spawned Claude subprocesses
- Use SDK's spawnClaudeCodeProcess option to capture PIDs via custom spawn
- Pass signal parameter to enable AbortController integration
- Wait for subprocess exit in deleteSession() with 5s timeout
- Escalate to SIGKILL if graceful exit fails
- Add orphan reaper running every 5 minutes as safety net

Files changed:
- src/services/worker/ProcessRegistry.ts (new): PID registry and reaper
- src/services/worker/SDKAgent.ts: Use custom spawn to capture PIDs
- src/services/worker/SessionManager.ts: Verify subprocess exit on delete
- src/services/worker-service.ts: Start/stop orphan reaper

Fixes #737

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

* fix: address code review feedback

- Replace busy-wait polling with event-based proc.once('exit')
- Detect and warn about multiple processes per session (race condition)

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

---------

Co-authored-by: bigphoot <bigphoot@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/services/worker-service.ts

@@ -69,6 +69,9 @@ import { SearchRoutes } from './worker/http/routes/SearchRoutes.js';
 import { SettingsRoutes } from './worker/http/routes/SettingsRoutes.js';
 import { LogsRoutes } from './worker/http/routes/LogsRoutes.js';
 
+// Process management for zombie cleanup (Issue #737)
+import { startOrphanReaper, reapOrphanedProcesses } from './worker/ProcessRegistry.js';
+
 /**
  * Build JSON status output for hook framework communication.
  * This is a pure function extracted for testability.
@@ -121,6 +124,9 @@ export class WorkerService {
   private initializationComplete: Promise<void>;
   private resolveInitialization!: () => void;
 
+  // Orphan reaper cleanup function (Issue #737)
+  private stopOrphanReaper: (() => void) | null = null;
+
   constructor() {
     // Initialize the promise that will resolve when background initialization completes
     this.initializationComplete = new Promise((resolve) => {
@@ -303,6 +309,16 @@ export class WorkerService {
       this.resolveInitialization();
       logger.info('SYSTEM', 'Background initialization complete');
 
+      // Start orphan reaper to clean up zombie processes (Issue #737)
+      this.stopOrphanReaper = startOrphanReaper(() => {
+        const activeIds = new Set<number>();
+        for (const [id] of this.sessionManager['sessions']) {
+          activeIds.add(id);
+        }
+        return activeIds;
+      });
+      logger.info('SYSTEM', 'Started orphan reaper (runs every 5 minutes)');
+
       // Auto-recover orphaned queues (fire-and-forget with error logging)
       this.processPendingQueues(50).then(result => {
         if (result.sessionsStarted > 0) {
@@ -404,6 +420,12 @@ export class WorkerService {
    * Shutdown the worker service
    */
   async shutdown(): Promise<void> {
+    // Stop orphan reaper before shutdown (Issue #737)
+    if (this.stopOrphanReaper) {
+      this.stopOrphanReaper();
+      this.stopOrphanReaper = null;
+    }
+
     await performGracefulShutdown({
       server: this.server.getHttpServer(),
       sessionManager: this.sessionManager,