Feat/add antigravity

[kenisteward]

I used AGY to generate this and it currently looks to support it. I would like someone to look over this as i'm testing the full phase 4 IMPL still.

Summary

This PR adds full platform support for the Google Antigravity (agy) agent, enabling developers to use bmalph workflows alongside Antigravity's native SKILL.md system.

Key Additions:

• Platform Definition (src/platform/antigravity.ts): Registers Antigravity, sets AGENTS.md as the root instruction file, and maps bmalph's command delivery to output native Antigravity skills into the .agents/skills/ directory using command frontmatter.
• Auto-Detection (src/platform/detect.ts): Added .agents and AGENTS.md to the strong detection markers so bmalph init will automatically select Antigravity in compatible workspaces.
• Ralph Driver (ralph/drivers/antigravity.sh): Added the execution driver that maps the .ralph/@fix_plan.md payload directly into the agy --prompt invocation to bridge Phase 4 implementations.
• Registry & CLI: Updated src/platform/registry.ts and src/cli.ts to expose the new platform option.
• Tests & Config: Updated the platform registry tests to expect 8 platforms, updated CHANGELOG.md, and bumped the version to 2.12.0.

Checklist

• Tests pass (npm test)
• Linting passes (npm run lint)
• Build succeeds (npm run build)
• Changelog updated (if applicable)
• Version bumped in package.json (if applicable)

[jdar]

The "--prompt" flag is not a valid parameter for agy.

This code is syntactically valid but cannot possibly work.

Suggest dogfooding before resubmitting

[kenisteward]

That in the first line of the pr description. I'm currently using it for an app to test things now. I wanted to make sure that the base skills were in place and made sense from and outside perspective.

I think my issue ( why I hadn't ran into a problem yet) is I'm in the prd phase of testing which bmalph doesn't care about