fix: prefer vim over nano for $EDITOR fallback (CLI + TUI)

prompt_toolkit's default editor list is: $VISUAL, $EDITOR, /usr/bin/editor,
/usr/bin/nano, /usr/bin/pico, /usr/bin/vi, /usr/bin/emacs — so when
neither env var is set, the base CLI launched nano. The TUI fell back
to a literal 'vi'. Same Ctrl+G keystroke, two different editors.

Pick the same chain on both surfaces:
  $VISUAL → $EDITOR → vim → vi → nano

CLI: override input_area.buffer._open_file_in_editor on the TextArea
once at app build time. Local to that buffer; doesn't touch
os.environ or affect other subprocesses.

TUI: extract resolveEditor() into ui-tui/src/lib/editor.ts. PATH walk
with accessSync(X_OK), no shelling out. Six-line unit test verifies
the priority order and the multi-entry PATH walk.

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

@@ -0,0 +1,66 @@
+import { chmodSync, mkdirSync, mkdtempSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { delimiter, join } from 'node:path'
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+
+import { resolveEditor } from './editor.js'
+
+describe('resolveEditor', () => {
+  let dir: string
+
+  const exe = (name: string) => {
+    const path = join(dir, name)
+    writeFileSync(path, '#!/bin/sh\nexit 0\n')
+    chmodSync(path, 0o755)
+
+    return path
+  }
+
+  beforeEach(() => {
+    dir = mkdtempSync(join(tmpdir(), 'editor-test-'))
+  })
+
+  afterEach(() => {
+    // tmp dir is small; let the OS reap it
+  })
+
+  it('honors $VISUAL above all else', () => {
+    expect(resolveEditor({ EDITOR: 'vim', PATH: dir, VISUAL: 'helix' })).toBe('helix')
+  })
+
+  it('falls back to $EDITOR when $VISUAL is unset', () => {
+    expect(resolveEditor({ EDITOR: 'nvim', PATH: dir })).toBe('nvim')
+  })
+
+  it('prefers vim over vi over nano on $PATH', () => {
+    exe('nano')
+    exe('vi')
+    const vim = exe('vim')
+
+    expect(resolveEditor({ PATH: dir })).toBe(vim)
+  })
+
+  it('falls back to vi when only vi and nano exist', () => {
+    exe('nano')
+    const vi = exe('vi')
+
+    expect(resolveEditor({ PATH: dir })).toBe(vi)
+  })
+
+  it('returns literal "vi" when nothing on PATH and no env', () => {
+    mkdirSync(join(dir, 'empty'), { recursive: true })
+
+    expect(resolveEditor({ PATH: join(dir, 'empty') })).toBe('vi')
+  })
+
+  it('walks multi-entry PATH', () => {
+    const a = mkdtempSync(join(tmpdir(), 'editor-a-'))
+    const b = mkdtempSync(join(tmpdir(), 'editor-b-'))
+
+    writeFileSync(join(b, 'vim'), '#!/bin/sh\n')
+    chmodSync(join(b, 'vim'), 0o755)
+
+    expect(resolveEditor({ PATH: [a, b].join(delimiter) })).toBe(join(b, 'vim'))
+  })
+})