Replace the chat-printed session list with a modal picker:
- SessionsPickerScreen lists only navi_code-profile sessions (filtered by
  settings.default_profile_id), with a filter input, a top "✚ New session"
  row, and the current session marked. Up/Down + Enter to pick, Esc cancels.
- /sessions pushes the screen. Selecting an existing session runs
  _switch_session_worker (persist via StateManager + attach_session, which
  replays history and restarts the WS bridge — no chat_panel.clear() after,
  since load_history already replaces the chat and clearing would erase the
  replayed history). The "New session" row runs _create_new_session_worker.

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