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-question.spec.ts

@@ -13,7 +13,7 @@ async function harness(): Promise<{ ctx: Context; api: ApiProxy }> {
   await ctx.plugin(UserInteractionService)
   return {
     ctx,
-    api: createApiProxy(ctx, { provider: 'p', model: 'm', cwd: '/tmp', workspaceRoot: '/tmp' }),
+    api: createApiProxy(ctx, { provider: 'p', model: 'm', cwd: '/tmp' }),
   }
 }