Add a status filter to dispatch watch

[jongio]

dispatch watch can report attention states, but hooks and dashboards often need only one state, such as waiting or interrupted.

Acceptance criteria:

• dispatch watch --status waiting --once only counts and lists waiting sessions.
• Streaming mode only prints transitions whose new status matches the filter.
• Supported values match existing attention labels: waiting, working, thinking, active, stale, compacting, interrupted, and idle.