fix(host,client): require fully qualified browse paths; clear the picker kind on close

ds-review-bot round 4. On Windows, isAbsolute admits rooted drive-less
forms (\foo, /foo) that resolve() then rebases onto the process's current
drive; both browse primitives now gate on a fullyQualified check (drive
letter or UNC on win32, POSIX-absolute elsewhere) with a platform test
seam, per-platform unit cases, and the contract wording updated on the
seam, the backend README pair, and the error messages.

The picker-kind effect also kept a resolved 'dialog' across close, so a
backend swapped while the menu was closed could paint the stale entry for
one frame on reopen; the close arm now clears the state, pinned by a
reopen-under-pending-read race test.

packages/client/ui-workspace/tests/workspace-picker.spec.tsx

@@ -300,6 +300,20 @@ describe('WorkspacePicker', () => {
     expect(screen.queryByRole('menuitem', { name: 'Open local folder…' })).toBeNull()
   })
 
+  it('clears the advertised kind on close so a reopen cannot paint the previous host entry', async () => {
+    const directoryPickerKind = vi.fn<() => Promise<string>>()
+      .mockImplementationOnce(async () => 'dialog')
+      // The reopened read never settles: the assertion below sees the paint
+      // that precedes any fresh answer.
+      .mockImplementation(() => new Promise<string>(() => {}))
+    const t = togglable(directoryPickerKind)
+    await screen.findByRole('menuitem', { name: 'Open local folder…' })
+    t.setOpen(false)
+    t.setOpen(true)
+    await screen.findByRole('menuitem', { name: 'Create a new workspace' })
+    expect(screen.queryByRole('menuitem', { name: 'Open local folder…' })).toBeNull()
+  })
+
   it('discards a stale describe failure after a newer open already answered', async () => {
     let rejectFirst!: (reason: Error) => void
     const first = new Promise<string>((_settle, reject) => { rejectFirst = reject })