Add Kubernetes operator patterns to instruction file

[varunreddy]

Summary

Extend the existing kubernetes.md instruction file with operator patterns (Custom Resource Definitions, controller-runtime, reconciliation loops).

Acceptance Criteria

• Add a section on CRD design and operator SDK patterns to examples/registry/instructions/kubernetes.md
• Include decision tree entries for when to use an operator vs a Helm chart
• Add anti-patterns for operator development
• Update composability hints to reference the new patterns
• All existing tests still pass (pytest)