Adds the files_mcp/ sibling package and core/path_guard.py which resolves arbitrary input paths and rejects any that escape the FILES_MCP_ROOT sandbox (handles .., ~, symlinks, NUL, empty). Adds tests/unit/test_path_guard.py covering accept, escape modes, symlink escape, and _init_root failure paths. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>