Align gatectl with the control-tool family

[cnberry]

Summary

• add shared repository guidance, agent skill, and just workflow
• document the four-tool family and the language-neutral script/install deployment contract
• preserve the proven guarded open/close implementation and safety model
• record the future Rust compatibility requirements without coupling private automation to Python

Verification

• 21 unit tests
• Ruff format and lint
• detect-secrets baseline scan
• installer syntax and executable checks
• wheel build, CLI help, Markdown link, and privacy checks