feat(boost): add Antigravity agent with guidelines and skills support

[soleinjast]

What’s new

This PR introduces Google Antigravity as a supported agent in Laravel Boost.

It adds first-class support for:

• Guidelines (AGENT.md) with Antigravity-specific transformations
• Workspace skills via .agent/skills, aligned with Antigravity’s documented context model

This allows Boost users to integrate Antigravity into existing CLI and workflow automation in the same way as other supported agents.

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Implementation details

• Adds a new Antigravity agent implementation
• Supports:
  • Project guidelines (AGENT.md)
  • Workspace skills (.agent/skills)
• Maps MCP configuration to Antigravity’s global config location
  (~/.gemini/antigravity/mcp_config.json) without attempting full MCP automation, as MCP support is still evolving for Antigravity
• Includes a small guideline transformation to handle Antigravity’s parsing behavior

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Backward compatibility

• No existing agents or workflows are affected
• No breaking changes

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Related discussion

Closes / relates to: #540

[pushpak1300]

I’m a bit torn, but my leaning is: until AntiGravity supports project-level MCP, we probably shouldn’t add support on our side, since it could confuse end users.

That said, AntiGravity already supports the .agent standard, so if someone installs the Opencode, the AntiGravity skill and guideline support should get enabled automatically without extra setup.

Net: I’d prefer we hold off on merging this for now, at least until project-level MCP is supported.

[stdrew]

@soleinjast 👊 thx for the additions even if on hold for now.
@pushpak1300 new Laravel fan & sry for requesting w/o PR. good catch on shotty/non-workspace MCP. as it happens, the rush of excitement for AG is already cooling down…AI hype cycles are wild.

still a v nice idea…if Google decides to keep caring about it (coin flip ☺️)