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

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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/shared/SettingsDefaultsManager.ts

@@ -52,6 +52,8 @@ export interface SettingsDefaults {
   CLAUDE_MEM_CONTEXT_SHOW_LAST_SUMMARY: string;
   CLAUDE_MEM_CONTEXT_SHOW_LAST_MESSAGE: string;
   CLAUDE_MEM_FOLDER_CLAUDEMD_ENABLED: string;
+  // Process Management
+  CLAUDE_MEM_MAX_CONCURRENT_AGENTS: string;  // Max concurrent Claude SDK agent subprocesses (default: 2)
   // Exclusion Settings
   CLAUDE_MEM_EXCLUDED_PROJECTS: string;  // Comma-separated glob patterns for excluded project paths
   CLAUDE_MEM_FOLDER_MD_EXCLUDE: string;  // JSON array of folder paths to exclude from CLAUDE.md generation
@@ -110,6 +112,8 @@ export class SettingsDefaultsManager {
     CLAUDE_MEM_CONTEXT_SHOW_LAST_SUMMARY: 'true',
     CLAUDE_MEM_CONTEXT_SHOW_LAST_MESSAGE: 'false',
     CLAUDE_MEM_FOLDER_CLAUDEMD_ENABLED: 'false',
+    // Process Management
+    CLAUDE_MEM_MAX_CONCURRENT_AGENTS: '2',  // Max concurrent Claude SDK agent subprocesses
     // Exclusion Settings
     CLAUDE_MEM_EXCLUDED_PROJECTS: '',  // Comma-separated glob patterns for excluded project paths
     CLAUDE_MEM_FOLDER_MD_EXCLUDE: '[]',  // JSON array of folder paths to exclude from CLAUDE.md generation