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-subagents.spec.ts

@@ -88,7 +88,7 @@ function bench(options: {
   ctx.provide('sessionProjections', { snapshot, restore, onChanged: () => () => {} })
   ctx.provide('userInteraction', { registerProvider: () => () => {} })
   const api = createApiProxy(ctx, {
-    provider: 'p', model: 'm', cwd: '/tmp', workspaceRoot: '/tmp',
+    provider: 'p', model: 'm', cwd: '/tmp',
   })
   return { api, getAgent, listChildren, inspect, snapshot, restore, followup, parent }
 }