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/styles.css

@@ -187,6 +187,26 @@ body {
   font-family: 'SF Mono', Monaco, monospace;
 }
 
+.header-system-stats {
+  display: flex;
+  gap: 0.75rem;
+  font-size: 0.7rem;
+  font-family: 'SF Mono', Monaco, monospace;
+  color: var(--text-dim);
+  padding: 0 0.5rem;
+  border-left: 1px solid var(--border);
+  margin-left: 0.5rem;
+}
+
+.header-system-stats span {
+  white-space: nowrap;
+}
+
+.header-system-stats .high {
+  color: var(--red);
+  font-weight: 500;
+}
+
 .btn-icon-header {
   display: flex;
   align-items: center;