[codex] migrate Gemini native CLI to Antigravity

[kulvirgit]

Summary

• Replace native Gemini CLI execution paths with Antigravity CLI (agy) while preserving the stable gemini model id.
• Update Claude command templates, Codex skill references, setup docs, and Codex defaults to use agy --sandbox -p.
• Treat bare gemini command values as legacy configs and add .antigravitycli/ to .gitignore because live Antigravity runs create local metadata.
• Bump plugin/config metadata to v1.7.0 and document the migration in the changelog.

Validation

• Confirmed installed agy supports -p, --sandbox, --add-dir, and --continue via agy --help.
• Live checked agy --sandbox -p ..., repeated --add-dir, and multiline -p "$(cat prompt.md)" prompts.
• Found agy --continue -p can prepend prior output, so convergence now uses a fresh agy --sandbox -p prompt.
• Ran JSON validation with jq empty.
• Ran git diff --check.
• Searched for stale executable Gemini paths: no active gemini -p, gemini --resume, command -v gemini, or "command": "gemini" remain.