fix: avoid event-loop stalls from synchronous tmux/ps calls (#100)

The stats collector (~2s) and mouse-mode sync (5s) ran execSync (pgrep/ps/
list-panes, 5s timeout each) per session on the server's single thread,
blocking the event loop. With several sessions or a momentarily slow tmux this
froze port 3000 for seconds-to-tens-of-seconds while the process stayed alive
and other ports were unaffected — self-healing, so it never restarted and the
60s loopback healthcheck missed it. Convert these hot-path calls to execAsync.

Also add an always-on event-loop lag monitor (utils/event-loop-monitor.ts) that
logs stalls >=1s to the web log, so this otherwise-invisible class of incident
leaves a quantified, timestamped trace.

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>

src/utils/index.ts

@@ -9,6 +9,8 @@
 export { BufferAccumulator } from './buffer-accumulator.js';
 export { CleanupManager } from './cleanup-manager.js';
 export { Debouncer, KeyedDebouncer } from './debouncer.js';
+export { startEventLoopMonitor } from './event-loop-monitor.js';
+export type { EventLoopMonitorHandle } from './event-loop-monitor.js';
 export { StaleExpirationMap } from './stale-expiration-map.js';
 export {
   ANSI_ESCAPE_PATTERN_FULL,