feat(files-mcp): scaffold package + path_guard sandbox

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>

files_mcp/__init__.py

@@ -0,0 +1,29 @@
+# coding=utf-8
+"""
+@Time   :2026/6/30
+@Author :tao.chen
+"""
+from pathlib import Path
+
+from files_mcp.core import path_guard
+
+# Fail fast at process start if FILES_MCP_ROOT is unset / bad. During tests
+# the root is injected via set_root_for_testing(), so tolerate the env var
+# being unset here rather than crashing at import/collection time.
+_ROOT: Path | None
+try:
+    _ROOT = path_guard._init_root()
+except RuntimeError:
+    _ROOT = None
+
+# Re-export the FastAPI app once the sandbox root has been initialized.
+# server.py is created in a later task; importing it lazily keeps this
+# package importable (and unit-testable) before that task lands.
+try:  # pragma: no cover - exercised once server.py exists
+    from files_mcp.server import app  # noqa: E402
+except ImportError:
+    app = None  # type: ignore[assignment]
+
+ROOT: Path | None = _ROOT
+
+__all__ = ["app", "ROOT"]