Tests for CLI documentation extension from tmuxp#1010

Biome-formatted CSS with consolidated light mode headerlink selectors

- Add argparse_exemplar to Sphinx extensions
- Add argparse-highlight.css to html_css_files
- Add mypy overrides for sphinx_argparse_neo modules
- Exclude tests/docs from mypy strict checking

Replace tmuxp-specific examples with inline test module creation
that works in any repository without external dependencies

Change `conftest\.py$` to `docs/_ext/conftest\.py$` to type-check root
conftest.py while avoiding duplicate module error from mypy_path config.

- Add create_parser() function to g module for documentation and --version
- Create docs/cli/index.md using sphinx-argparse directive
- Add cli/index to docs toctree
- Support --version/-V flag in CLI

- feat(docs): Render argparse metadata as semantic definition list
  Replace pipe-separated format with <dl>/<dt>/<dd> structure
- style(css): Use var(--code-font-size) for all argparse elements
  Add font-size to inline roles, pre.argparse-usage, .argparse-argument-name
  Add new .argparse-argument-meta flexbox styles
- fix(lexer): Include underscores in lowercase metavar patterns
  Change [a-z][-a-z0-9]* to [a-z][-a-z0-9_]* in 6 locations
- test(lexer): Add underscore metavar regression test
  Add fixtures and test for socket_name, config_file patterns

The previous implementation used argparse's version action which calls
sys.exit(), preventing run() from returning in test mode. Now version
is printed manually and returns None when G_IS_TEST is set, matching
the pattern used when no VCS is found.

Adds tests for --version and -V flags.

The documentation previously stated "all arguments are passed directly"
which was misleading since --version/-V is intercepted by g itself.
Updated epilog and documentation to accurately describe this behavior.

Furo's "auto" theme follows system prefers-color-scheme. The meta tag
dark styling only targeted explicit data-theme="dark", missing the
auto-dark case where body:not([data-theme="light"]) applies.

The light mode override for headerlink colors used body:not([data-theme="dark"])
which incorrectly matches auto mode even when system is dark. Split into
explicit light selector plus media query for auto + system light.

- CLI: g --version/-V now displays g's version
- Documentation: Add CLI documentation page
- Documentation: Linkable anchors for CLI arguments

Replace tmuxp examples in tests with generic "myapp".