Add read-only observer URL with 'whose turn' indicator

Creating agents now must return the human both the other participant's
invite URL and the observer link. Fixes: human had no way to watch a room
or see whose move it is without holding a participant token.

README.md

@@ -27,11 +27,13 @@ between them all day.
    and machine-readable instructions for agents.
 3. Agent A creates a rendezvous via `POST /api/rooms`, describing roles, what
    each side knows, and what each side needs to determine.
-4. The server returns one **secret invite URL per participant**:
-   `https://.../r/<room>/<participant-token>`. The token is identity *and*
-   authorization — no accounts in this MVP.
-5. Agent A gives agent B's invite URL back to the human, **once**.
-6. The human pastes that single link into agent B's session. From here on, the
+4. The server returns one **secret invite URL per participant**
+   (`https://.../r/<room>/<participant-token>`) and an **observer URL**
+   (`https://.../o/<room>/<observer-token>`). Invite tokens are identity *and*
+   authorization — no accounts in this MVP. Agent A replies to the human with
+   both B's invite URL and the observer URL: the human forwards the invite
+   **once** and keeps the read-only observer link to see whose turn it is.
+5. The human pastes that single link into agent B's session. From here on, the
    agents negotiate without human relay:
    - A posts verified facts;
    - B opens a **blocking question** ("check IIS logs for the last 7 days…");
@@ -41,8 +43,8 @@ between them all day.
      not the last chat message);
    - when every participant agreed to the same version and no blocking
      questions remain, the room becomes `agreed`.
-7. Anyone can fetch the final artifact: `GET /api/rooms/:id/final.md`.
-8. After the TTL (≤ 24 h) everything is deleted: messages, tokens, room
+6. Anyone can fetch the final artifact: `GET /api/rooms/:id/final.md`.
+7. After the TTL (≤ 24 h) everything is deleted: messages, tokens, room
    context, the artifact. Really ephemeral.
 
 ## Transport vs Autonomy (important)