refactor: drop the create-by-name workspace route

The Web picker collapsed onto the directory flow (see the one-route-to-add-a-workspace
Agent Note), leaving workspace.create({ name }) with no product consumer. Delete the
whole feed line: the wire schema's name member and WorkspaceApi spelling, the gateway's
workspaceRoot config/default and the mkdir branch, the client seam that carried the name
(WorkspaceCreateInput, WorkspacesService.create, intentName), the dsh web
--workspace-root flag, and the fixture's name handling. workspace-name-conflict stays as
workspace.rename's duplicate-title error.

packages/host/apiproxy/tests/api-proxy-config.spec.ts

@@ -24,7 +24,7 @@ import type { RpcRequest, RpcResponse } from '../src/api/rpc.ts'
 import { RpcId } from '../src/api/rpc.ts'
 import { createApiProxy } from '../src/api-proxy.ts'
 
-const DEFAULTS = { provider: 'p', model: 'm', cwd: '/tmp', workspaceRoot: '/tmp' }
+const DEFAULTS = { provider: 'p', model: 'm', cwd: '/tmp' }
 
 let nextRpc = 1
 function request<P>(payload: P): RpcRequest<P> {