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-search.spec.ts

@@ -27,7 +27,7 @@ vi.mock('node:fs/promises', async (importOriginal) => {
 })
 
 const sid = (value: string): SessionId => value as SessionId
-const defaults = { provider: 'p', model: 'm', cwd: '/tmp', workspaceRoot: '/tmp' }
+const defaults = { provider: 'p', model: 'm', cwd: '/tmp' }
 
 function request(query: string): RpcRequest<{ query: string }> {
   return { rpcId: RpcId(`search-${query}`), payload: { query } }