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/composer-draft-scroll.e2e.ts

@@ -232,7 +232,7 @@ describe('web e2e: composer draft scrolling', () => {
     tripwire = watchConsole(page)
     await page.goto(scaffold.baseUrl, { waitUntil: 'load' })
     await page.waitForSelector('[class*="frame"]', { timeout: 30_000 })
-    await connectFreshWorkspace(page, 'composer-draft-scroll')
+    await connectFreshWorkspace(page, scaffold.workspaceCwd, 'composer-draft-scroll')
     await page.locator('textarea:enabled').first().fill(DRAFT)
   }, 180_000)