docs: slash-command catalog section

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

README.md

@@ -100,6 +100,20 @@ Then `openclaw gateway restart`.
   `members` (list the channel's member userIds) · `join` (this agent
   joins) · `leave` (this agent leaves).
 
+## Slash-command catalog
+
+On `gateway_start` the plugin syncs OpenClaw's native-command catalog to
+each guild (`command-sync.ts`: `listNativeCommandSpecsForConfig` +
+`findCommandByNativeName`, dynamic arg `choices` snapshotted via
+`resolveCommandArgChoices`) → `PUT /api/commands`. This is exactly the data
+Discord registers as slash commands; Fabric's frontend uses it for `/`
+autocomplete. Fabric deliberately does **not** advertise the
+`nativeCommands` channel capability — it stays a text-command surface, so a
+`/<cmd>` message is delivered normally and OpenClaw's command system
+executes it (text-command + command session + auth), reusing the standard
+inbound path. Only `/no-reply` and `/force-proceed` stay
+server-intercepted by the guild.
+
 ## Install / build
 
 ```bash