Add Google Antigravity support to lat init

Summary

• Add Google Antigravity as a supported agent in lat init
• Antigravity uses AGENTS.md as its primary config (shared with other non-Claude agents)
• Writes workspace rules to .agents/rules/lat.md with CLI command references
• Installs the lat-md authoring skill at .agents/skills/lat-md/SKILL.md
• Updates legacy GEMINI.md if it already exists (not created from scratch)
• No per-workspace MCP server — Antigravity only supports global MCP config

What gets created

Design decisions

• No MCP setup: Antigravity doesn't support per-project MCP configuration, so all tool access is via CLI commands (lat search, lat check, etc.)
• No GEMINI.md creation: Antigravity is migrating from GEMINI.md to AGENTS.md. Legacy GEMINI.md is updated if present, not created fresh.
• Rules use AGENTS.md template (not cursor-rules): Since there's no MCP server, rules reference CLI commands instead of MCP tools.
• No command style needed: Without MCP or hooks, the binary path style choice doesn't apply.

Test plan

• Run lat init, select Antigravity — verify AGENTS.md, .agents/rules/lat.md, .agents/skills/lat-md/SKILL.md are created
• Run lat init in a project with existing GEMINI.md — verify it gets updated with markers
• Run lat init in a project without GEMINI.md — verify it's not created
• Run lat check — all checks pass
• npx tsc --noEmit passes