refactor: remove Smithery deployment mode

Smithery is no longer a supported deployment mode. Remove all Smithery-specific
code paths, middleware, configuration, and tests. This simplifies the codebase
by eliminating DeploymentMode enum, SmitheryConfigMiddleware, session config
context variables, and the smithery_main entrypoint.

Files deleted: Dockerfile.smithery, smithery.yaml, smithery_main.py
ADR-016 retained with deprecated status for historical reference.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/api/management.py

@@ -218,8 +218,6 @@ async def get_server_status(request: Request) -> JSONResponse:
         auth_mode = "multi_user_basic"
     elif mode == AuthMode.SINGLE_USER_BASIC:
         auth_mode = "basic"
-    elif mode == AuthMode.SMITHERY_STATELESS:
-        auth_mode = "smithery"
     else:
         auth_mode = "unknown"