This commit addresses a security vulnerability where unsanitized user inputs for commit_hash and file_path were passed directly to git commands in CheckpointManager.restore() and diff(). It validates commit hashes to be strictly hexadecimal characters without leading dashes (preventing flag injection like '--patch') and enforces file paths to stay within the working directory via root resolution. Regression tests test_restore_rejects_argument_injection, test_restore_rejects_invalid_hex_chars, and test_restore_rejects_path_traversal were added.