Implement Kubernetes provider

## Description
Implement full Kubernetes provider (currently a stub).

Run code execution in Kubernetes pods for scalable, cloud-native workloads.

## Requirements
- [ ] Connect to Kubernetes cluster (kubeconfig)
- [ ] Create pods with appropriate runtime images
- [ ] Execute code in pods
- [ ] Stream stdout/stderr via pod logs
- [ ] File system operations via kubectl cp
- [ ] Pod cleanup on stop
- [ ] Support for custom pod templates
- [ ] Resource limits (CPU, memory)

## References
- Current stub: `internal/provider/kubernetes/kubernetes.go`
- client-go: https://github.com/kubernetes/client-go