feat: rewrite GreenNode CLI in Go

[tvt286]

Summary

• Rewrite entire CLI from Python to Go
• Single binary (~10MB), zero runtime dependencies
• All 13 VKS commands + 4 configure commands preserved
• Remove all Python code (grncli/, tests/, setup.py, etc.)
• Update all docs, workflows, CLAUDE.md for Go

Highlights

• cobra CLI framework (same as kubectl, docker, gh)
• Cross-compile: linux/darwin/windows × amd64/arm64
• Startup time: ~10ms (was ~300ms Python)
• Distribution: download binary → run (no pip/Python needed)

Type of change

• Breaking change
• New feature
• Documentation

Checklist

• All 13 VKS commands working with real API
• Configure commands working (list, get, set, interactive)
• Output formats working (JSON, table, text, JMESPath)
• Auto-pagination, dry-run, delete confirmation, waiter all working
• Workflows updated for Go (build + test, release with cross-compile)
• All docs updated (README, CLAUDE.md, CONTRIBUTING, GitHub Pages)