fix(copilot): register companion PTY in session store and hide from UI

The hidden companion terminal for agent tabs was not registered in the
session store during init/polling, causing TouchBar key presses to be
silently dropped. Additionally, hidden sessions were added to tabOrder,
making them visible in the side panel and session hub.

- Register companion PTY as hidden session during init and polling loops
  (matching handleNewSessionCreated behavior)
- Exclude hidden sessions from tabOrder in addSession() so they never
  appear in tabs, side panel, or session hub
- Prevent hidden sessions from becoming activeId automatically

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dorlugasigal