cleanup(web): give adding a Workspace one route

Both Workspace surfaces offered "Open local folder…" and "Create a new
workspace" for one outcome. The browse occupant already carries its own
New folder affordance, so picking a directory covered creating one; the
name dialog only added a second vocabulary and a create target the
operator could neither see nor choose.

The surviving entry is named after the outcome — "Add workspace…" — and a
menu now appears only where there is something to choose between: with no
Workspace listed (the add-only sidebar header, or an empty hero list) the
anchor gesture raises the directory flow directly instead of a one-row
popover. An empty list counts as final only after the list baseline lands,
and a composition with no directory-flow occupant hides the sidebar button
rather than offering a dead one.

WorkspaceCreateFlow becomes WorkspacePickFlow (createOnly -> addOnly) and
the injected createWorkspace narrows to { path }. The host's
workspace.create({ name }) branch and `dsh web --workspace-root` lost their
last product consumer; both are marked at the call site for a follow-up.

packages/client/ui-workspace/tests/apply.spec.ts

@@ -102,8 +102,8 @@ describe('ui-workspace apply', () => {
     expect(b.rename).toHaveBeenCalledWith('ws', 'renamed')
     await browser.insertSessionBefore('ws' as never, 's1' as never, 's2' as never)
     expect(b.insertSessionBefore).toHaveBeenCalledWith('ws', 's1', 's2')
-    await browser.createWorkspace({ name: 'project' })
-    expect(b.create).toHaveBeenCalledWith({ name: 'project' })
+    await browser.createWorkspace({ path: '/tmp/browser-project' })
+    expect(b.create).toHaveBeenCalledWith({ path: '/tmp/browser-project' })
 
     const picker = (b.slots.entries('conversation.hero.workspace')[0]!.inject as () => WorkspacePickerInjected)()
     await picker.createWorkspace({ path: '/tmp/project' })