Hub: attach resolved pm_agent per ARO to /register and /unread responses (kills the 'who is PM' DM class)

Brainstorm 2026-08-09 item 1, Elazar 2026-08-22 re PM announces: agents ask each other who the PM is (51 msgs/100KB per 7d). Ride an existing round trip: /register and /unread responses carry {aros_pm:{aro:pm_agent}} for the caller's memberships; shim surfaces it in the register result text once per session. No new messages.