fix: avoid prompt_toolkit complex tempfile bug and prefer nvim first

Setting buffer.tempfile = 'prompt.md' pushed prompt_toolkit into its
complex-tempfile path, which creates a temp dir and then calls
os.makedirs() on that same path when no subdirectory is present. That
raises EEXIST before the editor can launch.

Keep prompt_toolkit on the simple tempfile path with .md suffix, and
make the editor fallback chain explicit on both surfaces:
$VISUAL -> $EDITOR -> nvim -> vim -> vi -> nano.

ui-tui/src/lib/editor.test.ts

@@ -33,12 +33,13 @@ describe('resolveEditor', () => {
     expect(resolveEditor({ EDITOR: 'nvim', PATH: dir })).toBe('nvim')
   })
 
-  it('prefers vim over vi over nano on $PATH', () => {
+  it('prefers nvim over vim over vi over nano on $PATH', () => {
     exe('nano')
     exe('vi')
-    const vim = exe('vim')
+    exe('vim')
+    const nvim = exe('nvim')
 
-    expect(resolveEditor({ PATH: dir })).toBe(vim)
+    expect(resolveEditor({ PATH: dir })).toBe(nvim)
   })
 
   it('falls back to vi when only vi and nano exist', () => {