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', () => {

ui-tui/src/lib/editor.ts

@@ -6,7 +6,7 @@ import { delimiter, join } from 'node:path'
  *
  * Order of preference:
  *   1. $VISUAL / $EDITOR (user's explicit choice)
- *   2. first executable found on $PATH from `vim` → `vi` → `nano`
+ *   2. first executable found on $PATH from `nvim` → `vim` → `vi` → `nano`
  *   3. literal `'vi'` so spawnSync still has something to try
  *
  * Mirrors the override on `input_area.buffer._open_file_in_editor` in cli.py
@@ -14,7 +14,7 @@ import { delimiter, join } from 'node:path'
  * doesn't surprise the user with nano in one and vim in the other.
  */
 export function resolveEditor(env: NodeJS.ProcessEnv = process.env): string {
-  return env.VISUAL || env.EDITOR || findExecutable(env.PATH ?? '', 'vim', 'vi', 'nano') || 'vi'
+  return env.VISUAL || env.EDITOR || findExecutable(env.PATH ?? '', 'nvim', 'vim', 'vi', 'nano') || 'vi'
 }
 
 function findExecutable(path: string, ...names: string[]): null | string {