fix(windows-editor): default EDITOR=notepad so /edit and Ctrl+X Ctrl+E work

Pre-existing Windows bug surfaced while reviewing the portable-MinGit
install: prompt_toolkit's Buffer.open_in_editor() falls back to POSIX
absolute paths (/usr/bin/nano, /usr/bin/vi, /usr/bin/emacs) that don't
exist on native Windows.  When neither $EDITOR nor $VISUAL is set,
Ctrl+X Ctrl+E ("open prompt in editor") and /edit both silently do
nothing on Windows — the user hits the key, nothing happens, no error.

This wasn't caused by MinGit (full Git for Windows doesn't fix it either,
because the Windows Python subprocess call resolves `/usr/bin/nano` as
`C:\usr\bin\nano`, which doesn't exist even with nano installed).

Fixes:
- hermes_cli/stdio.py::configure_windows_stdio now sets EDITOR=notepad
  on Windows if neither EDITOR nor VISUAL is set.  notepad.exe is in
  every Windows install, works as a blocking editor (subprocess.call
  waits for the window to close), and writes back to the file.
- hermes_cli/config.py (hermes config edit): reorder fallback list so
  Windows tries notepad first — previously nano led the list, which
  required Git Bash / WSL to be in PATH.
- Users who want VSCode / Neovim / Notepad++ can still override via
  $env:EDITOR — that's checked before our default kicks in.  Docstring
  spells out the common overrides.

The Ink TUI (`hermes --tui`) already handled Windows correctly via
ui-tui/src/lib/editor.ts falling back to notepad.exe on win32 — this
commit brings the classic prompt_toolkit CLI into parity.

3 new tests in test_windows_native_support.py verify:
- EDITOR=notepad gets set when unset on Windows
- Explicit $EDITOR is respected
- $VISUAL is respected (not overwritten by our default)

hermes_cli/config.py

@@ -4696,11 +4696,19 @@ def edit_config():
     
     # Find editor
     editor = os.getenv('EDITOR') or os.getenv('VISUAL')
-    
+
     if not editor:
-        # Try common editors
-        for cmd in ['nano', 'vim', 'vi', 'code', 'notepad']:
-            import shutil
+        # Try common editors — order is platform-aware so Windows users
+        # land on a working editor (notepad) even without Git Bash or nano
+        # installed.  On POSIX, prefer nano/vim over code/notepad because
+        # it's more likely to be present on headless / server systems.
+        import shutil
+        import sys as _sys
+        if _sys.platform == "win32":
+            candidates = ['notepad', 'code', 'vim', 'vi', 'nano']
+        else:
+            candidates = ['nano', 'vim', 'vi', 'code', 'notepad']
+        for cmd in candidates:
             if shutil.which(cmd):
                 editor = cmd
                 break