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/web/public/index.html

@@ -35,6 +35,10 @@
           <span class="status-text">Connecting</span>
         </div>
         <div class="header-tokens" id="headerTokens" title="Total tokens across all sessions">0 tokens</div>
+        <div class="header-system-stats" id="headerSystemStats" title="System resource usage">
+          <span class="stat-cpu" id="statCpu">CPU: --</span>
+          <span class="stat-mem" id="statMem">Mem: --</span>
+        </div>
         <button class="btn-icon-header" onclick="app.openAppSettings()" title="App Settings">&#x2699;</button>
       </div>
     </header>