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/ProcessRegistry.ts

@@ -19,6 +19,7 @@
 import { spawn, exec, ChildProcess } from 'child_process';
 import { promisify } from 'util';
 import { logger } from '../../utils/logger.js';
+import { cleanupExcessChromaProcesses } from '../infrastructure/ProcessManager.js';
 
 const execAsync = promisify(exec);
 
@@ -212,7 +213,7 @@ async function killSystemOrphans(): Promise<number> {
 
   try {
     const { stdout } = await execAsync(
-      'ps -eo pid,ppid,args 2>/dev/null | grep -E "claude.*haiku|claude.*output-format" | grep -v grep'
+      'ps -eo pid,ppid,args 2>/dev/null | grep -E "claude.*haiku|claude.*output-format|chroma-mcp" | grep -v grep'
     );
 
     let killed = 0;
@@ -262,6 +263,9 @@ export async function reapOrphanedProcesses(activeSessionIds: Set<number>): Prom
   // Daemon children: find idle SDK processes that didn't terminate
   killed += await killIdleDaemonChildren();
 
+  // Count-based: kill excess chroma-mcp processes regardless of age
+  killed += await cleanupExcessChromaProcesses();
+
   return killed;
 }