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>
