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)

tests/tools/test_windows_native_support.py

@@ -73,6 +73,8 @@ class TestConfigureWindowsStdio:
         monkeypatch.delenv("PYTHONIOENCODING", raising=False)
         monkeypatch.delenv("PYTHONUTF8", raising=False)
         monkeypatch.delenv("HERMES_DISABLE_WINDOWS_UTF8", raising=False)
+        monkeypatch.delenv("EDITOR", raising=False)
+        monkeypatch.delenv("VISUAL", raising=False)
 
         reconfigure_calls = []
 
@@ -86,14 +88,51 @@ class TestConfigureWindowsStdio:
 
         monkeypatch.setattr(stdio, "_reconfigure_stream", fake_reconfigure)
         monkeypatch.setattr(stdio, "_flip_console_code_page_to_utf8", fake_flip)
+        # Pretend notepad.exe is on PATH (it always is on real Windows hosts,
+        # but not on the Linux CI runner — mock it so the editor default
+        # survives).
+        monkeypatch.setattr(stdio, "_default_windows_editor", lambda: "notepad")
 
         result = stdio.configure_windows_stdio()
         assert result is True
         assert os.environ.get("PYTHONIOENCODING") == "utf-8"
         assert os.environ.get("PYTHONUTF8") == "1"
+        # EDITOR must be set so prompt_toolkit's open_in_editor finds
+        # a working program on Windows (it defaults to /usr/bin/nano).
+        assert os.environ.get("EDITOR") == "notepad"
         assert len(cp_calls) == 1  # SetConsoleOutputCP path hit
         assert len(reconfigure_calls) == 3  # stdout, stderr, stdin
 
+    def test_respects_existing_editor_var(self, monkeypatch):
+        """User's explicit EDITOR wins over our default."""
+        from hermes_cli import stdio
+
+        monkeypatch.setattr(stdio, "is_windows", lambda: True)
+        monkeypatch.setenv("EDITOR", "code --wait")
+        monkeypatch.setattr(stdio, "_reconfigure_stream", lambda *a, **kw: None)
+        monkeypatch.setattr(stdio, "_flip_console_code_page_to_utf8", lambda: None)
+        monkeypatch.setattr(stdio, "_default_windows_editor", lambda: "notepad")
+
+        stdio.configure_windows_stdio()
+        assert os.environ["EDITOR"] == "code --wait"
+
+    def test_respects_existing_visual_var(self, monkeypatch):
+        """VISUAL takes precedence over our EDITOR default too."""
+        from hermes_cli import stdio
+
+        monkeypatch.setattr(stdio, "is_windows", lambda: True)
+        monkeypatch.delenv("EDITOR", raising=False)
+        monkeypatch.setenv("VISUAL", "nvim")
+        monkeypatch.setattr(stdio, "_reconfigure_stream", lambda *a, **kw: None)
+        monkeypatch.setattr(stdio, "_flip_console_code_page_to_utf8", lambda: None)
+        monkeypatch.setattr(stdio, "_default_windows_editor", lambda: "notepad")
+
+        stdio.configure_windows_stdio()
+        # EDITOR should NOT be set when VISUAL already is (prompt_toolkit
+        # checks VISUAL first anyway, but we also shouldn't override it).
+        assert os.environ.get("EDITOR", "") != "notepad"
+        assert os.environ["VISUAL"] == "nvim"
+
     def test_respects_existing_env_var(self, monkeypatch):
         """User's explicit PYTHONIOENCODING wins over our default."""
         from hermes_cli import stdio