feat: configurable subprocess pool limit for SDK agents (#995)

* feat: configurable subprocess pool limit for SDK agents

Prevents runaway accumulation of Claude SDK agent subprocesses by
enforcing a configurable concurrency limit.

- New CLAUDE_MEM_MAX_CONCURRENT_AGENTS setting (default: 2)
- Promise-based waitForSlot() in ProcessRegistry (not polling per
  review feedback on #830)
- Waiters are notified via unregisterProcess when a slot frees up
- SDKAgent.startSession() waits for a slot before spawning
- 60s timeout prevents indefinite waits

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

* fix: remove unused originalUnregister const and getActiveCount import

Cleanup from Greptile review:
- Remove dead `originalUnregister` variable in ProcessRegistry
- Remove unused `getActiveCount` import in SDKAgent

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Happy <yesreply@happy.engineering>

src/services/worker/SDKAgent.ts

@@ -21,7 +21,7 @@ import { buildIsolatedEnv, getAuthMethodDescription } from '../../shared/EnvMana
 import type { ActiveSession, SDKUserMessage } from '../worker-types.js';
 import { ModeManager } from '../domain/ModeManager.js';
 import { processAgentResponse, type WorkerRef } from './agents/index.js';
-import { createPidCapturingSpawn, getProcessBySession, ensureProcessExit } from './ProcessRegistry.js';
+import { createPidCapturingSpawn, getProcessBySession, ensureProcessExit, waitForSlot } from './ProcessRegistry.js';
 
 // Import Agent SDK (assumes it's installed)
 // @ts-ignore - Agent SDK types may not be available
@@ -88,6 +88,11 @@ export class SDKAgent {
       session.forceInit = false;
     }
 
+    // Wait for agent pool slot (configurable via CLAUDE_MEM_MAX_CONCURRENT_AGENTS)
+    const settings = SettingsDefaultsManager.loadFromFile(USER_SETTINGS_PATH);
+    const maxConcurrent = parseInt(settings.CLAUDE_MEM_MAX_CONCURRENT_AGENTS, 10) || 2;
+    await waitForSlot(maxConcurrent);
+
     // Build isolated environment from ~/.claude-mem/.env
     // This prevents Issue #733: random ANTHROPIC_API_KEY from project .env files
     // being used instead of the configured auth method (CLI subscription or explicit API key)