Add sagemaker-ai plugin (re-list after YAML fix)

sagemaker-ai was dropped from the marketplace in #1762 (validate-plugins
adoption) due to a manifest/YAML error. AWS has since fixed it; the plugin
validates clean at awslabs/agent-plugins@187edde (claude plugin validate passes).

Re-listed as a git-subdir source SHA-pinned to current monorepo HEAD,
matching its sibling AWS entries (deploy-on-aws, databases-on-aws).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -2119,6 +2119,19 @@
         }
       }
     },
+    {
+      "name": "sagemaker-ai",
+      "description": "Build, train, and deploy AI models with deep AWS AI/ML expertise brought directly into your coding assistants, covering the surface area of Amazon SageMaker AI.",
+      "category": "development",
+      "source": {
+        "source": "git-subdir",
+        "url": "https://github.com/awslabs/agent-plugins.git",
+        "path": "plugins/sagemaker-ai",
+        "ref": "main",
+        "sha": "187edde6e122116b43211049195627a5069bda80"
+      },
+      "homepage": "https://github.com/awslabs/agent-plugins"
+    },
     {
       "name": "sanity",
       "description": "Sanity content platform integration with MCP server, agent skills, and slash commands. Query and author content, build and optimize GROQ queries, design schemas, and set up Visual Editing.",