Fix git_repo fixture for submodule file:// protocol (#511)

## Summary
- Add `set_home` dependency to `git_repo` fixture so child processes (e.g., spawned by `git submodule add`) can find `$HOME/.gitconfig` with `protocol.file.allow=always`
- Add regression test `test_git_repo_fixture_submodule_file_protocol`

## Root Cause
The `git_repo` fixture had `set_gitconfig` (which sets `GIT_CONFIG` env var) but not `set_home`. Child processes spawned by `git submodule add` don't inherit `GIT_CONFIG` and need `$HOME/.gitconfig` to find the protocol configuration.

Author: tony

CHANGES

@@ -20,6 +20,13 @@ $ uv add libvcs --prerelease allow
 _Notes on the upcoming release will go here._
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+### Tests
+
+- pytest plugin: Add `set_home` dependency to `git_repo` fixture (#511)
+
+  Follow-up to #510 for #509: Ensures child processes spawned by git submodule
+  operations can find `$HOME/.gitconfig` with `protocol.file.allow=always`.
+
 ## libvcs 0.38.3 (2026-01-25)
 
 ### Tests

src/libvcs/pytest_plugin.py

@@ -701,6 +701,7 @@ def git_repo(
     projects_path: pathlib.Path,
     git_remote_repo: pathlib.Path,
     set_gitconfig: pathlib.Path,
+    set_home: None,  # Needed for child processes (e.g. submodules)
 ) -> GitSync:
     """Pre-made git clone of remote repo checked out to user's projects dir."""
     remote_repo_name = unique_repo_name(remote_repos_path=projects_path)

tests/test_pytest_plugin.py

@@ -16,6 +16,7 @@
     import pathlib
 
     from libvcs.pytest_plugin import CreateRepoPytestFixtureFn
+    from libvcs.sync.git import GitSync
 
 
 @pytest.mark.skipif(not shutil.which("git"), reason="git is not available")
@@ -258,3 +259,55 @@ def test_gitconfig_submodule_file_protocol(
     # Verify submodule was actually added
     gitmodules = main_repo / ".gitmodules"
     assert gitmodules.exists(), "Submodule should create .gitmodules file"
+
+
+@pytest.mark.skipif(not shutil.which("git"), reason="git is not available")
+def test_git_repo_fixture_submodule_file_protocol(
+    git_repo: GitSync,
+    create_git_remote_repo: CreateRepoPytestFixtureFn,
+    git_commit_envvars: dict[str, str],
+    user_path: pathlib.Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Test that git_repo fixture allows file:// protocol for submodule operations.
+
+    This validates that the git_repo fixture has proper HOME setup so child
+    processes (spawned by git submodule add) can find $HOME/.gitconfig with
+    protocol.file.allow=always.
+
+    The git_repo fixture depends on set_home to ensure child processes
+    (like git clone spawned by git submodule add) can find the test gitconfig.
+
+    See: https://github.com/vcs-python/libvcs/issues/509
+    """
+    from libvcs.pytest_plugin import git_remote_repo_single_commit_post_init
+
+    # Verify that HOME is set to user_path where test gitconfig resides
+    assert os.environ.get("HOME") == str(user_path), (
+        f"git_repo fixture should set HOME to user_path.\n"
+        f"Expected: {user_path}\n"
+        f"Actual: {os.environ.get('HOME')}\n"
+        "git_repo fixture is missing set_home dependency"
+    )
+
+    # Block system config to prevent interference
+    monkeypatch.setenv("GIT_CONFIG_SYSTEM", os.devnull)
+
+    # Create a repo to use as submodule source (with a commit so it can be cloned)
+    submodule_source = create_git_remote_repo()
+    git_remote_repo_single_commit_post_init(
+        remote_repo_path=submodule_source,
+        env=git_commit_envvars,
+    )
+
+    # Add submodule - this spawns child git clone that needs HOME set correctly
+    # NOTE: We do NOT use the local config workaround here
+    result = git_repo.cmd.submodules.add(
+        repository=f"file://{submodule_source}",
+        path="vendor/lib",
+    )
+
+    assert "fatal" not in result.lower(), (
+        f"git submodule add failed: {result}\n"
+        "git_repo fixture needs set_home dependency for child processes"
+    )