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>

dist/fabric/src/fabric-client.js

@@ -68,6 +68,13 @@ export class FabricClient {
     joinChannel(guildEndpoint, guildToken, channelId) {
         return this.post(`${guildEndpoint}/api/channels/${channelId}/join`, {}, guildToken);
     }
+    leaveChannel(guildEndpoint, guildToken, channelId) {
+        return this.post(`${guildEndpoint}/api/channels/${channelId}/leave`, {}, guildToken);
+    }
+    // [{ userId, bypass }] — bypass is true only for discuss/work bypass-list
+    channelMembers(guildEndpoint, guildToken, channelId) {
+        return this.req('GET', `${guildEndpoint}/api/channels/${channelId}/members`, guildToken);
+    }
     // ---- channel canvas (one pinned doc per channel) ----
     canvasUrl(endpoint, channelId) {
         return `${endpoint}/api/channels/${channelId}/canvas`;

dist/fabric/src/tools.js

@@ -163,4 +163,43 @@ export function registerFabricTools(api, client, identity) {
             }
         },
     }));
+    // fabric-channel: channel membership (one tool, three actions).
+    api.registerTool((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) => {
+            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)}` };
+            }
+        },
+    }));
 }