feat(mcp): Add EKS node diagnostics MCP

Port the sample-eks-node-diagnostics-mcp project into
mcp/aws-eks-node-diagnostics-mcp/, including CDK infrastructure,
Lambda sources, diagnostic runbooks, CNI semantics, the packaged
skill, and tests.

The skill archive (skills/eks-node-diagnostics.zip) is force-added
because the repo's root .gitignore (*.zip) would otherwise exclude
it.