fix: async I/O, map iteration bug, unified cleanup, Zod validation, error handling

- Convert sync readFileSync/statSync to async in subagent-watcher.ts to
  unblock event loop on hot paths (transcript reads, liveness checks)
- Fix Map mutation during iteration in closeSessionLogViewerWindows and
  closeSessionImagePopups (collect IDs first, then iterate to close)
- Unify session cleanup into shared _cleanupSessionData() method called
  from both closeSession() and session:deleted handler to prevent leaks
- Add Zod validation schemas for 20+ API routes that used raw type casts
- Add consecutive error tracking (5 errors/60s triggers exit for systemd
  restart) and SIGHUP handler for SSH disconnect safety

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli.ts

@@ -525,6 +525,7 @@ program
       };
       process.on('SIGTERM', () => shutdown('SIGTERM'));
       process.on('SIGINT', () => shutdown('SIGINT'));
+      process.on('SIGHUP', () => shutdown('SIGHUP'));
     } catch (err) {
       console.error(chalk.red(`✗ Failed to start web server: ${getErrorMessage(err)}`));
       process.exit(1);