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/tools.ts

@@ -199,4 +199,48 @@ export function registerFabricTools(
       }
     },
   }));
+
+  // fabric-channel: channel membership (one tool, three actions).
+  api.registerTool((ctx: Ctx) => ({
+    name: 'fabric-channel',
+    description:
+      'Channel membership. action: members (list channel member userIds) | ' +
+      'join (this agent joins the channel) | leave (this agent leaves).',
+    parameters: {
+      type: 'object',
+      additionalProperties: false,
+      required: ['action', 'guildNodeId', 'channelId'],
+      properties: {
+        action: { type: 'string', enum: ['members', 'join', 'leave'] },
+        guildNodeId: { type: 'string' },
+        channelId: { type: 'string' },
+      },
+    },
+    execute: async (p: {
+      action: 'members' | 'join' | 'leave';
+      guildNodeId: string;
+      channelId: string;
+    }) => {
+      const agentId = ctx.agentId;
+      if (!agentId) return { ok: false, error: 'no agent context' };
+      const { guild, token } = await ctxGuild(agentId, p.guildNodeId);
+      const ep = guild.endpoint;
+      switch (p.action) {
+        case 'members': {
+          const members = await client.channelMembers(ep, token, p.channelId);
+          return { ok: true, members };
+        }
+        case 'join': {
+          await client.joinChannel(ep, token, p.channelId);
+          return { ok: true, joined: true };
+        }
+        case 'leave': {
+          await client.leaveChannel(ep, token, p.channelId);
+          return { ok: true, left: true };
+        }
+        default:
+          return { ok: false, error: `unknown action ${String(p.action)}` };
+      }
+    },
+  }));
 }