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.

cli.py

@@ -9790,6 +9790,28 @@ class HermesCLI:
         # complex-tempfile path takes care of cleanup via shutil.rmtree.
         input_area.buffer.tempfile = 'prompt.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
+        # 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('vim')
+                or shutil.which('vi')
+                or shutil.which('nano')
+            )
+            if not chosen:
+                return False
+            try:
+                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
         # wrapping of long lines so the input area always fits its content.
         def _input_height():