feat: Implement ADR-016 Smithery stateless deployment mode

Adds support for Smithery hosted deployment with stateless operation:

- Add DeploymentMode enum with SELF_HOSTED and SMITHERY_STATELESS modes
- Add get_deployment_mode() to detect mode from SMITHERY_DEPLOYMENT env var
- Update get_client() to create per-request clients from session config
- Add conditional tool registration (skip semantic search in Smithery mode)
- Add conditional /app admin UI mounting (skip in Smithery mode)
- Create smithery.yaml with configSchema for user credentials
- Create Dockerfile.smithery for minimal stateless container
- Create smithery_main.py entrypoint for Smithery deployment

In Smithery mode:
- Users provide nextcloud_url, username, app_password via session config
- Each request creates a fresh NextcloudClient (no state between requests)
- Semantic search tools are disabled (no vector database)
- Admin UI (/app) is disabled (no webhooks, vector viz)

All existing self-hosted functionality remains unchanged.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/config.py

@@ -2,8 +2,37 @@ import logging
 import logging.config
 import os
 from dataclasses import dataclass
+from enum import Enum
 from typing import Any, Optional
 
+
+class DeploymentMode(Enum):
+    """Deployment mode for the MCP server.
+
+    SELF_HOSTED: Full features, environment-based configuration.
+                 Supports vector sync, semantic search, admin UI.
+
+    SMITHERY_STATELESS: Stateless mode for Smithery hosting.
+                        Session-based configuration, no persistent storage.
+                        Excludes semantic search, vector sync, admin UI.
+    """
+
+    SELF_HOSTED = "self_hosted"
+    SMITHERY_STATELESS = "smithery"
+
+
+def get_deployment_mode() -> DeploymentMode:
+    """Detect deployment mode from environment.
+
+    Returns:
+        DeploymentMode.SMITHERY_STATELESS if SMITHERY_DEPLOYMENT=true,
+        otherwise DeploymentMode.SELF_HOSTED (default).
+    """
+    if os.getenv("SMITHERY_DEPLOYMENT", "false").lower() == "true":
+        return DeploymentMode.SMITHERY_STATELESS
+    return DeploymentMode.SELF_HOSTED
+
+
 LOGGING_CONFIG = {
     "version": 1,
     "disable_existing_loggers": False,