feat: add TUI dashboard extension for at-a-glance system status

Renders a persistent widget above the editor showing:
- Pi version (with update indicator if behind latest npm)
- Slack bridge status (live HTTP probe)
- Session health (control-agent, sentry-agent, dev-agents)
- Todo stats (active/done/total)
- Worktree count
- Current model and uptime

Refreshes every 30s with zero LLM token cost. Admin can attach
to the running baudbot tmux session and see health without
sending any messages.

Also adds /dashboard command for immediate refresh.

Author: Baudbot