perf: optimize for long-running Claude sessions

Critical fixes (unbounded growth):
- Add 64KB line buffer limit with 100ms periodic flush in session.ts
- Add task tracker cleanup (max 100 completed tasks)
- Add scheduled runs auto-cleanup after 1 hour
- Fix session leaks in scheduled run iterations

High priority (performance):
- Add SSE event batching (50ms output, 100ms task updates)
- Parallelize screen stats with Promise.all()
- Add 1MB buffer limit to respawn controller

Medium priority (frontend):
- Make xterm scrollback configurable (default 5000)
- Debounce renderSessionTabs() at 100ms

New feature:
- Add CPU and memory usage display in frontend header
- New /api/system/stats endpoint

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

src/task-tracker.ts

@@ -1,5 +1,8 @@
 import { EventEmitter } from 'node:events';
 
+// Maximum number of completed tasks to keep in memory
+const MAX_COMPLETED_TASKS = 100;
+
 /**
  * Represents a background task spawned by Claude Code
  */
@@ -159,12 +162,54 @@ export class TaskTracker extends EventEmitter {
       } else {
         this.emit('taskCompleted', task);
       }
+
+      // Clean up old completed tasks to prevent unbounded growth
+      this.cleanupCompletedTasks();
     }
 
     // Clean up pending
     this.pendingToolUses.delete(toolUseId);
   }
 
+  /**
+   * Remove old completed/failed tasks when exceeding the limit
+   * Keeps running tasks and the most recent completed tasks
+   */
+  private cleanupCompletedTasks(): void {
+    const completedTasks: BackgroundTask[] = [];
+
+    // Collect all completed/failed tasks
+    for (const task of this.tasks.values()) {
+      if (task.status === 'completed' || task.status === 'failed') {
+        completedTasks.push(task);
+      }
+    }
+
+    // If under limit, no cleanup needed
+    if (completedTasks.length <= MAX_COMPLETED_TASKS) {
+      return;
+    }
+
+    // Sort by end time (oldest first)
+    completedTasks.sort((a, b) => (a.endTime || 0) - (b.endTime || 0));
+
+    // Remove oldest tasks beyond the limit
+    const toRemove = completedTasks.slice(0, completedTasks.length - MAX_COMPLETED_TASKS);
+    for (const task of toRemove) {
+      // Remove from parent's children list if applicable
+      if (task.parentId) {
+        const parent = this.tasks.get(task.parentId);
+        if (parent) {
+          const childIndex = parent.children.indexOf(task.id);
+          if (childIndex !== -1) {
+            parent.children.splice(childIndex, 1);
+          }
+        }
+      }
+      this.tasks.delete(task.id);
+    }
+  }
+
   private createTaskFromTerminal(agentType: string, context: string): void {
     const taskId = `terminal-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`;
     const parentId = this.taskStack.length > 0 ? this.taskStack[this.taskStack.length - 1] : null;