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/screen-manager.ts

@@ -228,16 +228,19 @@ export class ScreenManager extends EventEmitter {
     }
   }
 
-  // Get all screens with stats
+  // Get all screens with stats (parallel for better performance)
   async getScreensWithStats(): Promise<ScreenSessionWithStats[]> {
-    const result: ScreenSessionWithStats[] = [];
+    const screens = Array.from(this.screens.values());
 
-    for (const screen of this.screens.values()) {
-      const stats = await this.getProcessStats(screen.sessionId);
-      result.push({ ...screen, stats: stats || undefined });
-    }
+    // Fetch all stats in parallel
+    const statsPromises = screens.map(screen => this.getProcessStats(screen.sessionId));
+    const allStats = await Promise.all(statsPromises);
 
-    return result;
+    // Combine screens with their stats
+    return screens.map((screen, i) => ({
+      ...screen,
+      stats: allStats[i] || undefined
+    }));
   }
 
   // Start periodic stats collection