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>

main.py

@@ -10,6 +10,7 @@ from fastapi import FastAPI
 
 from common.factory import init_mcp_server
 from common.logging import logger
+from files_mcp import app as files_app
 from spark_executor import app as spark_executor_app
 
 
@@ -39,6 +40,11 @@ MCP_SERVICES: list[McpService] = [
         app=spark_executor_app,
         mount_path="/spark-executor-mcp",
     ),
+    McpService(
+        name="files",
+        app=files_app,
+        mount_path="/files-mcp",
+    ),
 ]