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.

apps/web/tests/seeded-history.e2e.ts

@@ -42,7 +42,7 @@ describe('web e2e: seeded history renders through cold resume', () => {
 
   beforeAll(async () => {
     scaffold = await launchWebScaffold({})
-    // The workspace-aware flow runs sessions in <workspaceRoot>/workspace
+    // The workspace-aware flow runs sessions in <workspaceCwd>/workspace
     // (the composer's default draft name); the read-tool targets must live in
     // that session cwd. Pre-creating the directory is safe: create-by-name
     // adopts an existing directory.