Add bot-driven room app UI via im.paarrot.ui state.

Bots can publish a declarative panel UI that takes over a room; user
actions are sent as im.paarrot.ui.action timeline events.

docs/README.md

@@ -62,6 +62,7 @@ New feature? Copy [`handoff/TEMPLATE.md`](./handoff/TEMPLATE.md) into `handoff/f
 | Sub-rooms | [sub-rooms](./handoff/features/sub-rooms/HANDOFF.md) |
 | Lobby (space card view) | [lobby-forums](./handoff/features/lobby-forums/HANDOFF.md) |
 | Forum spaces (post feed UI) | [forum](./handoff/features/forum/HANDOFF.md) |
+| Room App UI (bot-driven) | [room-app-ui](./handoff/features/room-app-ui/HANDOFF.md) |
 
 ### Settings & customization