pytest(gitconfig) Avoid always reusing gitconfig (#512)

## Summary

- Fix `gitconfig` fixture to always write config - removes early return that skipped writing when file exists
- Follow-up to #510, #511 for #509

## Problem

The `gitconfig` fixture had an early return at lines 157-158:

```python
if gitconfig.exists():
    return gitconfig
```

If ANYTHING creates `.gitconfig` first (empty or incomplete) - git itself during `git init`, the nspawn container setup, another fixture - the fixture returns early WITHOUT writing `protocol.file.allow=always`.

Confirmed by reporter running debug script: empty `.gitconfig` was created by container/git before the fixture could write the proper config.

## Solution

Remove the early return. The fixture now always writes the complete config with `protocol.file.allow=always`.

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 0.38.5 (unreleased)
+
+### Bug Fixes
+
+- pytest plugin: Fix `gitconfig` fixture to always write config (#512)
+
+  Follow-up to #510, #511 for #509: The fixture was returning early if
+  `.gitconfig` already existed (even if empty or incomplete), skipping
+  the write of `protocol.file.allow=always`. Now always writes the full
+  config to ensure submodule operations work in strict build environments.
+
 ## libvcs 0.38.4 (2026-01-25)
 
 ### Tests

src/libvcs/pytest_plugin.py

@@ -154,9 +154,6 @@ def gitconfig(
     """Return git configuration, pytest fixture."""
     gitconfig = user_path / ".gitconfig"
 
-    if gitconfig.exists():
-        return gitconfig
-
     gitconfig.write_text(
         textwrap.dedent(
             f"""