Long-poll subscription that pushes ctx.info() on each new inbox file.
Replaces bash polling daemons (watch-relay-*.sh) with server-initiated push.
Call once at session start (e.g. via SessionStart hook). Server holds the
subscription, watches the calling agent's role-specific inbox dir, and
fires info-level notifications on each new relay file arrival. Client
re-calls this in a loop for persistent coverage.
Per PR #1 (CCR-inversion-for-relay-pickup): `inbox_filter` parameter added
to BYPASS role-based dir resolution. Use when role detection is unreliable
OR when subscribing to a specific canonical inbox (e.g., 'cc_tb').
Closes 3-week-old feedback_relay_arrival_invisible_midsession HARD RULE.
ConnectorNo auth