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>

dist/fabric/src/fabric-client.js

@@ -21,6 +21,24 @@ export class FabricClient {
         }
         return (await res.json());
     }
+    // Generic JSON request (GET/PUT/PATCH/DELETE). Empty 2xx body -> null
+    // (Fabric returns an empty body when a channel has no canvas).
+    async req(method, url, auth, body) {
+        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} ${text}`);
+        }
+        const text = await res.text();
+        return (text ? JSON.parse(text) : null);
+    }
     // Exchange an agent API key for a Fabric user session (+ guild tokens).
     agentLogin(apiKey) {
         return this.post(`${this.centerApiBase}/auth/agent/login`, { apiKey });
@@ -50,4 +68,24 @@ export class FabricClient {
     joinChannel(guildEndpoint, guildToken, channelId) {
         return this.post(`${guildEndpoint}/api/channels/${channelId}/join`, {}, guildToken);
     }
+    // ---- channel canvas (one pinned doc per channel) ----
+    canvasUrl(endpoint, channelId) {
+        return `${endpoint}/api/channels/${channelId}/canvas`;
+    }
+    // null when the channel has no canvas
+    getCanvas(endpoint, token, channelId) {
+        return this.req('GET', this.canvasUrl(endpoint, channelId), token);
+    }
+    // share / replace (caller becomes the sharer)
+    shareCanvas(endpoint, token, channelId, body) {
+        return this.req('PUT', this.canvasUrl(endpoint, channelId), token, body);
+    }
+    // update in place (original sharer only — else the guild returns 403)
+    updateCanvas(endpoint, token, channelId, body) {
+        return this.req('PATCH', this.canvasUrl(endpoint, channelId), token, body);
+    }
+    // remove ("close") the canvas (original sharer only)
+    removeCanvas(endpoint, token, channelId) {
+        return this.req('DELETE', this.canvasUrl(endpoint, channelId), token);
+    }
 }