Surface live background-task count in the prompt_toolkit status bar so users
can see at a glance that a /background task exists and is running — no need
to ask the agent about it (the agent has no visibility into bg sessions by
design).
- _get_status_bar_snapshot now reports active_background_tasks from len()
of the live _background_tasks dict (entries are removed in the task
thread's finally block, so this reflects truly-running tasks)
- Indicator shown only on medium (<76) and wide (>=76) tiers; narrow (<52)
stays minimal since it's already cramped
- No invalidate plumbing needed: status bar fragments are pulled via lambda
on every redraw, and the bg thread already calls _app.invalidate() on exit
Refs #8568
