feat: enhance /api/debug/memory endpoint with comprehensive metrics

- Add getStats() method to SubagentWatcher for internal resource tracking
- Return detailed Map sizes for all server and subagent Maps
- Track file watchers, directory watchers, and transcript watchers separately
- Track all timer types (respawn, idle, pending starts)
- Add totals for quick overview of resource usage
- Useful for debugging memory leaks and resource exhaustion

The enhanced endpoint now returns:
- Memory usage (heap, external, array buffers)
- Map sizes by component (server, subagent watcher)
- Watcher counts (file, directory, transcript)
- Timer counts by type
- Spawn agent statistics

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

src/subagent-watcher.ts

@@ -419,6 +419,36 @@ export class SubagentWatcher extends EventEmitter {
     return this.getProjectHash(workingDir);
   }
 
+  /**
+   * Get internal statistics for memory monitoring.
+   * Returns counts of internal Maps and resources.
+   */
+  getStats(): {
+    agentCount: number;
+    fileWatcherCount: number;
+    dirWatcherCount: number;
+    idleTimerCount: number;
+    pendingToolCallsCount: number;
+    knownDirsCount: number;
+    filePositionsCount: number;
+  } {
+    // Count pending tool calls across all agents
+    let pendingToolCallsCount = 0;
+    for (const agentCalls of this.pendingToolCalls.values()) {
+      pendingToolCallsCount += agentCalls.size;
+    }
+
+    return {
+      agentCount: this.agentInfo.size,
+      fileWatcherCount: this.fileWatchers.size,
+      dirWatcherCount: this.dirWatchers.size,
+      idleTimerCount: this.idleTimers.size,
+      pendingToolCallsCount,
+      knownDirsCount: this.knownSubagentDirs.size,
+      filePositionsCount: this.filePositions.size,
+    };
+  }
+
   /**
    * Get recent subagents (modified within specified minutes)
    */