feat: add Google Antigravity and Gemini CLI support

[ppraneth]

This PR integrates native support for Google Antigravity and the Gemini CLI by exposing the existing SMG Dev Guide skills as workflows.

Changes included:

• Created .agents/workflows/ directory containing explicit workflow .md files (verify-pr.md, implement-feature.md, review-pr.md, and map.md).
• These workflows wire up the existing SKILL.md instructions, exposing them as clickable /slash-commands directly in the Antigravity IDE UI.
• Updated README.md to document the new supported environments (Google Antigravity & Gemini CLI).
• Added project-level, global, and CLI installation instructions to the README.

Affected Skills

• smg:map
• smg:contribute
• smg:review-pr
• smg:implement
• Recipe files (list which)
• Command router (smg.md)
• Plugin config (plugin.json)

Checklist

• Follows conventional commit title format
• Commit(s) are signed off (git commit -s)
• Tested with claude plugins add . locally
• Skill triggers correctly on intended prompts
• No broken @file.md references in skills