docs: slash-command registry section

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -32,6 +32,12 @@ introspects the user/guild tokens Center issues.
 - **Channel canvas** — one pinned document per channel (`md`/`html`/`text`),
   re-share replaces, only the original sharer may update/remove; emits
   `canvas.updated` / `canvas.removed`.
+- **Slash-command registry** — guild-global catalog: `PUT /api/commands`
+  (the OpenClaw plugin syncs OpenClaw's native-command specs here),
+  `GET /api/commands` (frontend `/` autocomplete). Stored verbatim;
+  execution is unchanged (a `/<cmd>` message flows normally to the plugin →
+  OpenClaw command system; only `/no-reply`,`/force-proceed` are
+  server-intercepted).
 - **Realtime** — socket.io `/realtime`; `join_channel`/`leave_channel`,
   `message.created/updated/deleted`, `canvas.*`, presence, typing.