cmd/git(fix[Git.run]): Implement config_env parameter

why: The config_env parameter was accepted but never used, silently
ignoring callers who expected --config-env=<name>=<envvar> to be passed.
what:
- Add implementation to emit --config-env flag after config handling

Author: tony

src/libvcs/cmd/git.py

@@ -250,6 +250,8 @@ def stringify(v: t.Any) -> str:
 
             for k, v in config.items():
                 cli_args.extend(["--config", f"{k}={stringify(v)}"])
+        if config_env is not None:
+            cli_args.append(f"--config-env={config_env}")
         if git_dir is not None:
             cli_args.extend(["--git-dir", str(git_dir)])
         if work_tree is not None: