fix: derive canvas root from workspace dir when not explicitly configured

When canvasHost.root is not set, use <workspace>/canvas instead of
hardcoded ~/clawd/canvas, so the canvas directory respects the
agents.defaults.workspace setting.

src/config/types.gateway.ts

@@ -36,7 +36,7 @@ export type DiscoveryConfig = {
 
 export type CanvasHostConfig = {
   enabled?: boolean;
-  /** Directory to serve (default: ~/clawd/canvas). */
+  /** Directory to serve (default: <workspace>/canvas, e.g. ~/clawd/canvas). */
   root?: string;
   /** HTTP port to listen on (default: 18793). */
   port?: number;

src/gateway/server-runtime-state.ts

@@ -1,4 +1,5 @@
 import type { Server as HttpServer } from "node:http";
+import path from "node:path";
 import { WebSocketServer } from "ws";
 import { CANVAS_HOST_PATH } from "../canvas-host/a2ui.js";
 import { type CanvasHostHandler, createCanvasHostHandler } from "../canvas-host/server.js";
@@ -37,6 +38,7 @@ export async function createGatewayRuntimeState(params: {
   deps: CliDeps;
   canvasRuntime: RuntimeEnv;
   canvasHostEnabled: boolean;
+  defaultWorkspaceDir: string;
   allowCanvasHostInTests?: boolean;
   logCanvas: { info: (msg: string) => void; warn: (msg: string) => void };
   log: { info: (msg: string) => void; warn: (msg: string) => void };
@@ -75,7 +77,7 @@ export async function createGatewayRuntimeState(params: {
     try {
       const handler = await createCanvasHostHandler({
         runtime: params.canvasRuntime,
-        rootDir: params.cfg.canvasHost?.root,
+        rootDir: params.cfg.canvasHost?.root ?? path.join(params.defaultWorkspaceDir, "canvas"),
         basePath: CANVAS_HOST_PATH,
         allowInTests: params.allowCanvasHostInTests,
         liveReload: params.cfg.canvasHost?.liveReload,

src/gateway/server.impl.ts

@@ -301,6 +301,7 @@ export async function startGatewayServer(
     deps,
     canvasRuntime,
     canvasHostEnabled,
+    defaultWorkspaceDir,
     allowCanvasHostInTests: opts.allowCanvasHostInTests,
     logCanvas,
     log,