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.

cli.py

@@ -9783,23 +9783,24 @@ class HermesCLI:
                 completer=_completer,
             ),
         )
-        # Match the TUI's editor handoff: the temp file lands at
-        # <mkdtemp>/prompt.md so vim/nano/helix pick up markdown syntax
-        # highlighting from the .md extension and the title bar reads
-        # "prompt.md" instead of a random "tmpXXXXXX". prompt_toolkit's
-        # complex-tempfile path takes care of cleanup via shutil.rmtree.
-        input_area.buffer.tempfile = 'prompt.md'
+        # Keep prompt_toolkit on its simple tempfile path. Setting
+        # buffer.tempfile = "prompt.md" triggers its complex-tempfile branch,
+        # which tries to mkdir() the mkdtemp() directory again and raises
+        # EEXIST. The suffix keeps markdown highlighting without that bug.
+        input_area.buffer.tempfile_suffix = '.md'
 
         # prompt_toolkit's default fallback chain prefers /usr/bin/nano over
         # /usr/bin/vi when neither $VISUAL nor $EDITOR is set. The TUI's
-        # resolveEditor() prefers vim → vi → nano. Override this single
+        # resolveEditor() prefers nvim → vim → vi → nano. Override this single
         # buffer's resolver so both surfaces pick the same editor.
         import shlex
         import subprocess
+
         def _hermes_pick_editor(filename: str) -> bool:
             chosen = (
                 os.environ.get('VISUAL')
                 or os.environ.get('EDITOR')
+                or shutil.which('nvim')
                 or shutil.which('vim')
                 or shutil.which('vi')
                 or shutil.which('nano')
@@ -9810,6 +9811,7 @@ class HermesCLI:
                 return subprocess.call(shlex.split(chosen) + [filename]) == 0
             except OSError:
                 return False
+
         input_area.buffer._open_file_in_editor = _hermes_pick_editor
 
         # Dynamic height: accounts for both explicit newlines AND visual

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 {