pytest_plugin: Add GIT_CONFIG_GLOBAL for child processes (#513)

## Summary
- Adds `GIT_CONFIG_GLOBAL` to `set_gitconfig` fixture
- When git spawns child processes (e.g., `git submodule add`), `GIT_CONFIG` is explicitly excluded from the child environment
- `GIT_CONFIG_GLOBAL` IS inherited by child processes, fixing submodule operations in strict build environments

Follow-up to #512 / v0.38.5 for #509

Author: tony

CHANGES

@@ -20,6 +20,17 @@ $ uv add libvcs --prerelease allow
 _Notes on the upcoming release will go here._
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+## libvcs 0.38.6 (unreleased)
+
+### Bug Fixes
+
+- pytest plugin: Add `GIT_CONFIG_GLOBAL` to `set_gitconfig` fixture (#512)
+
+  Follow-up to v0.38.5 for #509: When git spawns child processes (e.g.,
+  during `git submodule add`), it explicitly excludes `GIT_CONFIG` from
+  the child environment. `GIT_CONFIG_GLOBAL` is the proper way to override
+  global config location as it IS inherited by child processes.
+
 ## libvcs 0.38.5 (2026-01-25)
 
 ### Bug Fixes

src/libvcs/pytest_plugin.py

@@ -180,6 +180,7 @@ def set_gitconfig(
 ) -> pathlib.Path:
     """Set git configuration."""
     monkeypatch.setenv("GIT_CONFIG", str(gitconfig))
+    monkeypatch.setenv("GIT_CONFIG_GLOBAL", str(gitconfig))  # For child processes
     return gitconfig