Refresh AWS plugins: add amplify/databases/sagemaker, remove migration

Adds three plugins from awslabs/agent-plugins:
- aws-amplify (development)
- databases-on-aws (database)
- sagemaker-ai (development)

Removes migration-to-aws (deprecated by the AWS team).

.claude-plugin/marketplace.json

@@ -470,19 +470,6 @@
       },
       "homepage": "https://github.com/awslabs/agent-plugins"
     },
-    {
-      "name": "datadog",
-      "description": "Use Datadog directly in Claude Code through a preconfigured Datadog MCP server. Query logs, metrics, traces, dashboards, and more through natural conversation. This plugin is in preview.",
-      "author": {
-        "name": "Datadog"
-      },
-      "category": "monitoring",
-      "source": {
-        "source": "url",
-        "url": "https://github.com/datadog-labs/claude-code-plugin.git"
-      },
-      "homepage": "https://www.datadoghq.com/"
-    },
     {
       "name": "dataverse",
       "description": "Agent skills for building on, analyzing, and managing Microsoft Dataverse — with Dataverse MCP, PAC CLI, and Python SDK.",