feat(files-mcp): mount at /files-mcp via MCP_SERVICES registry

- main.py: import files_app, append McpService entry to MCP_SERVICES.
  Lifespan already iterates the list, so no edit to the lifespan function.
- tests/conftest.py: autouse fixture rebinds files_mcp.core.path_guard._ROOT
  to a per-test tmp_path so the sandbox is fresh for every test.
- files_mcp/__init__.py: drop the try/except around the server import now
  that server.py exists, restoring fail-fast on a missing/corrupt server
  module in production. The _init_root() try/except stays, since pytest
  collection can import the package before FILES_MCP_ROOT is set.

Verified via uv run pytest: 327 tests pass, no regressions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/conftest.py

@@ -6,3 +6,20 @@ from pathlib import Path
 ROOT = Path(__file__).resolve().parents[1]
 if str(ROOT) not in sys.path:
     sys.path.insert(0, str(ROOT))
+
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _isolate_files_mcp_root(tmp_path, monkeypatch):
+    """Point files_mcp at a fresh tmp_path for every test.
+
+    Sets FILES_MCP_ROOT and rebinds files_mcp.core.path_guard's module-level
+    _ROOT, so each test starts with an empty sandbox. Spark_executor tests
+    are unaffected by this fixture — they don't touch the files_mcp sandbox.
+    """
+    monkeypatch.setenv("FILES_MCP_ROOT", str(tmp_path))
+    from files_mcp.core import path_guard
+    path_guard.set_root_for_testing(tmp_path.resolve())
+    yield