feat(plugin): fabric-channel tool (members / join / leave)

One tool, three actions backed by FabricClient channelMembers (GET
/channels/:id/members -> [{userId,bypass}]), joinChannel, and new
leaveChannel (POST /channels/:id/leave).

Verified: client-level smoke against the running guild — members
initial=[tester], after join echo2 present, after leave echo2 gone.

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

README.md

@@ -96,6 +96,9 @@ Then `openclaw gateway restart`.
   (create/replace; caller becomes sharer) · `update` (edit in place;
   sharer-only) · `close` (remove; sharer-only). `share` needs
   `title`/`format`(`md`|`html`|`text`)/`source`.
+- `fabric-channel` — channel membership; one tool, three `action`s:
+  `members` (list the channel's member userIds) · `join` (this agent
+  joins) · `leave` (this agent leaves).
 
 ## Install / build