feat: add onboarding CLI dependency setup

[bgdnvk]

Summary

• add clanker onboarding scan for provider CLI and provider-context detection with JSON output
• add clanker onboarding install and clanker onboarding agent-prompt so local agents can install missing provider CLIs after user approval
• include install commands, verify commands, docs URLs, and agent instructions for AWS, GCP, Azure, Cloudflare, DigitalOcean, Hetzner, Kubernetes, GitHub, Docker, Terraform, and OpenTofu

Verification

• go test ./...
• go build -o bin/clanker .
• ./bin/clanker onboarding scan --format json --provider aws
• ./bin/clanker onboarding install --dry-run --format json --provider cloudflare
• git diff --check