Add plan doc status to session status filter

Add "Has plan" as a selectable row in the Session Status Filter overlay (`!` key) alongside the existing attention statuses, so users can filter by plan doc presence from the same UI.

**Details**: [add-plan-doc-to-session-status-filter.md](https://github.com/jongio/dispatch/blob/main/docs/issues/add-plan-doc-to-session-status-filter.md)

Related: #21, #22