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>

smithery.yaml

@@ -0,0 +1,38 @@
+# Smithery configuration for Nextcloud MCP Server
+# See: https://smithery.ai/docs/build/configuration
+# ADR-016: Stateless deployment mode for multi-user public Nextcloud instances
+
+runtime: "container"
+
+build:
+  dockerfile: "Dockerfile.smithery"
+  dockerBuildPath: "."
+
+startCommand:
+  type: "http"
+  configSchema:
+    type: "object"
+    required:
+      - "nextcloud_url"
+      - "username"
+      - "app_password"
+    properties:
+      nextcloud_url:
+        type: "string"
+        title: "Nextcloud URL"
+        description: "Your Nextcloud instance URL (e.g., https://cloud.example.com). Must be publicly accessible."
+        pattern: "^https?://.+"
+      username:
+        type: "string"
+        title: "Username"
+        description: "Your Nextcloud username"
+        minLength: 1
+      app_password:
+        type: "string"
+        title: "App Password"
+        description: "Nextcloud app password. Generate at Settings > Security > App passwords. Do NOT use your main password."
+        minLength: 1
+  exampleConfig:
+    nextcloud_url: "https://cloud.example.com"
+    username: "alice"
+    app_password: "xxxxx-xxxxx-xxxxx-xxxxx-xxxxx"