fix: stream from current position, faster session detection (500ms)

- New sessions start from current file offset, not 0. Shows live
  thinking from the moment of detection, not a backlog dump.
- Session poll reduced from 2s to 500ms for faster pickup.
- Auto-pin with null body (MM pin API quirk).

src/session-monitor.js

@@ -34,7 +34,7 @@ class SessionMonitor extends EventEmitter {
   constructor(opts) {
     super();
     this.transcriptDir = opts.transcriptDir;
-    this.pollMs = opts.pollMs || 2000;
+    this.pollMs = opts.pollMs || 500;
     this.defaultChannel = opts.defaultChannel || null;
     this.logger = opts.logger || null;