pytest_plugin(docs[GitCommitEnvVars]): Add docstring to GitCommitEnvVars TypeAlias

why: sphinx's automodule :members: skips module-level TypeAliases whose
value has a __module__ that differs from the documented module (e.g.
dict[str, str] has __module__ = 'builtins').  Static source analysis via
ModuleAnalyzer.attr_docs only picks up the member when a docstring is
present.  Without it, GitCommitEnvVars never enters the py:data inventory
and cannot be cross-referenced from the fixture summary table.

what:
- Add one-line docstring to GitCommitEnvVars: TypeAlias so sphinx's
  static analyser includes it in automodule :members: output, registering
  it as a py:data entry in the Sphinx inventory

Author: tony

src/libvcs/pytest_plugin.py

@@ -267,6 +267,7 @@ def unique_repo_name(remote_repos_path: pathlib.Path, max_retries: int = 15) ->
 
 InitCmdArgs: t.TypeAlias = list[str] | None
 GitCommitEnvVars: t.TypeAlias = dict[str, str]
+"""Environment variable mapping passed to ``git commit`` subprocess calls."""
 Env: t.TypeAlias = _ENV