fix: prevent chroma-mcp spawn storm with 5-layer defense (641 processes → max 2)

During SIGHUP testing with 6+ active sessions, ChromaSync.ensureConnection()
had no mutex — concurrent fire-and-forget syncObservation() calls each spawned
a chroma-mcp subprocess via StdioClientTransport, creating 641 orphans in ~5min.
Error-driven reconnection formed a positive feedback loop amplifying the storm.

Defense layers:
- Layer 0: Connection mutex via promise memoization (prevents concurrent spawns)
- Layer 1: Pre-spawn process count guard using execFileSync('ps') (kills excess)
- Layer 2: Hardened close() with try-finally + Unix pkill in GracefulShutdown
- Layer 3: Count-based orphan reaper in ProcessManager (not age-based)
- Layer 4: Circuit breaker stops retries after 3 consecutive failures for 60s

Closes #1063, closes #695
Relates to #1010, #707

src/services/worker-service.ts

@@ -66,6 +66,7 @@ import {
   removePidFile,
   getPlatformTimeout,
   cleanupOrphanedProcesses,
+  cleanupExcessChromaProcesses,
   spawnDaemon,
   createSignalHandler
 } from './infrastructure/ProcessManager.js';
@@ -316,6 +317,7 @@ export class WorkerService {
   private async initializeBackground(): Promise<void> {
     try {
       await cleanupOrphanedProcesses();
+      await cleanupExcessChromaProcesses();
 
       // Load mode configuration
       const { ModeManager } = await import('./domain/ModeManager.js');