Add tempfile and improve git error handling

[MikeGarde]

• Add tempfile as a dev-dependency in Cargo.toml to enable temporary file handling in tests
• Update Cargo.lock to lock tempfile and its transitive dependencies for reproducible CI
• Improve src/git.rs error handling to include git's stderr when present, falling back to exit status if not
• Make staged_diff_for_file robust when run from subdirectories by anchoring the pathspec to the repo root with :/path; document this behavior
• Extend tests/git.rs with a focused test for staged_diff_for_file and staged_files from a subdirectory; add helper repo_with_staged_nested_file to create a temp repo with a nested staged change
• Update imports in tests to include staged_diff_for_file and staged_files to exercise staging-related functionality