feat(plugin): fabric-canvas tool; fabric-register env=AGENT_ID only

- bin/fabric-register.mjs: only AGENT_ID is read from the environment;
  --api-key is flag-only (no FABRIC_API_KEY); dropped FABRIC_CENTER_API_BASE
  / FABRIC_IDENTITY_FILE / OPENCLAW_PATH env fallbacks (flags + sensible
  defaults; --center still falls back to openclaw.json).
- New fabric-canvas tool (one tool, four actions): read / share / update /
  close the channel's single pinned canvas. Backed by FabricClient
  get/share/update/removeCanvas (GET/PUT/PATCH/DELETE; empty 2xx body ->
  null). update/close are sharer-only server-side.
- README updated.

Verified: client-level smoke against the running guild —
read(empty→null) → share(v1) → read → update(v2) → close(→null) all pass.

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

src/fabric-client.ts

@@ -29,6 +29,30 @@ export class FabricClient {
     return (await res.json()) as T;
   }
 
+  // Generic JSON request (GET/PUT/PATCH/DELETE). Empty 2xx body -> null
+  // (Fabric returns an empty body when a channel has no canvas).
+  private async req<T>(
+    method: string,
+    url: string,
+    auth?: string,
+    body?: unknown,
+  ): Promise<T> {
+    const res = await fetch(url, {
+      method,
+      headers: {
+        ...(body !== undefined ? { 'content-type': 'application/json' } : {}),
+        ...(auth ? { authorization: `Bearer ${auth}` } : {}),
+      },
+      body: body !== undefined ? JSON.stringify(body) : undefined,
+    });
+    if (!res.ok) {
+      const text = await res.text().catch(() => '');
+      throw new Error(`${method} ${url} -> ${res.status} ${text}`);
+    }
+    const text = await res.text();
+    return (text ? JSON.parse(text) : null) as T;
+  }
+
   // Exchange an agent API key for a Fabric user session (+ guild tokens).
   agentLogin(apiKey: string): Promise<FabricSession> {
     return this.post<FabricSession>(`${this.centerApiBase}/auth/agent/login`, { apiKey });
@@ -86,4 +110,57 @@ export class FabricClient {
   joinChannel(guildEndpoint: string, guildToken: string, channelId: string): Promise<unknown> {
     return this.post(`${guildEndpoint}/api/channels/${channelId}/join`, {}, guildToken);
   }
+
+  // ---- channel canvas (one pinned doc per channel) ----
+
+  private canvasUrl(endpoint: string, channelId: string): string {
+    return `${endpoint}/api/channels/${channelId}/canvas`;
+  }
+
+  // null when the channel has no canvas
+  getCanvas(
+    endpoint: string,
+    token: string,
+    channelId: string,
+  ): Promise<FabricCanvas | null> {
+    return this.req('GET', this.canvasUrl(endpoint, channelId), token);
+  }
+
+  // share / replace (caller becomes the sharer)
+  shareCanvas(
+    endpoint: string,
+    token: string,
+    channelId: string,
+    body: CanvasInput,
+  ): Promise<FabricCanvas> {
+    return this.req('PUT', this.canvasUrl(endpoint, channelId), token, body);
+  }
+
+  // update in place (original sharer only — else the guild returns 403)
+  updateCanvas(
+    endpoint: string,
+    token: string,
+    channelId: string,
+    body: Partial<CanvasInput>,
+  ): Promise<FabricCanvas> {
+    return this.req('PATCH', this.canvasUrl(endpoint, channelId), token, body);
+  }
+
+  // remove ("close") the canvas (original sharer only)
+  removeCanvas(endpoint: string, token: string, channelId: string): Promise<unknown> {
+    return this.req('DELETE', this.canvasUrl(endpoint, channelId), token);
+  }
 }
+
+export type CanvasFormat = 'md' | 'html' | 'text';
+export type CanvasInput = { title: string; format: CanvasFormat; source: string };
+export type FabricCanvas = {
+  channelId: string;
+  sharerUserId: string;
+  title: string;
+  format: CanvasFormat;
+  source: string;
+  version: number;
+  createdAt: string;
+  updatedAt: string;
+};