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-fork.spec.ts

@@ -85,7 +85,6 @@ const api = (ctx: Context) => createApiProxy(ctx, {
   provider: 'default-provider',
   model: 'default-model',
   cwd: '/tmp',
-  workspaceRoot: '/tmp',
 })
 
 describe('sessions.fork', () => {