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>

src/fabric-client.ts

@@ -111,6 +111,23 @@ export class FabricClient {
     return this.post(`${guildEndpoint}/api/channels/${channelId}/join`, {}, guildToken);
   }
 
+  leaveChannel(guildEndpoint: string, guildToken: string, channelId: string): Promise<unknown> {
+    return this.post(`${guildEndpoint}/api/channels/${channelId}/leave`, {}, guildToken);
+  }
+
+  // [{ userId, bypass }] — bypass is true only for discuss/work bypass-list
+  channelMembers(
+    guildEndpoint: string,
+    guildToken: string,
+    channelId: string,
+  ): Promise<Array<{ userId: string; bypass?: boolean }>> {
+    return this.req(
+      'GET',
+      `${guildEndpoint}/api/channels/${channelId}/members`,
+      guildToken,
+    );
+  }
+
   // ---- channel canvas (one pinned doc per channel) ----
 
   private canvasUrl(endpoint: string, channelId: string): string {