Operator feed v2: bidirectional operator↔daemon messaging (@-mention + room_post)

## v2 of the operator feed (#39) — messaging layer (NOT YET DISPATCHED) Linked follow-on to #39 (v1 = read-only feed + noise filter + needs-you lane + audible ping). This adds the bidirectional messaging so the operator can RESPOND from the room instead of camping in the raw orchestrator session. ## Scope 1. **Operator → daemon:** operator @-mentions a daemon in the room composer → the message is delivered to that daemon's inbox (delivery mechanism = the decision tracked on **#36 Q1** — reuse it, don't re-decide). The message also appears in the feed. 2. **Daemon → operator:** a new MCP tool `room_post` (requires `actor_ref`) lets a daemon post into its project's room → surfaced in the feed + the operator inbox, classified as needs_you when directed at the operator. 3. Messages persist (a room_messages table or reuse the inbox store) and render inline in the #39 feed. ## Depends on #39 v1 (the feed + classification + inbox surfacing must exist first). Also gated on #36 Q1 (daemon-delivery mechanism). ## Acceptance - Operator @-mention routes to the daemon inbox; daemon room_post surfaces in feed + inbox; messages persist + render in the feed. Tests for routing + persistence. Suite green; pint. Brief trailer with this id. ## Provenance Split from #39 at operator direction (2026-07-02): "keep the messaging as a linked v2." Held at idea until #39 v1 lands + #36 Q1 resolved.