feat: Add Google Antigravity CLI harness support

[mhenke]

Description

Implement support for the Antigravity CLI (agy) harness in the multi-harness agentic plugin marketplace.

Requirements

1. Output Directory: .antigravity/ (gitignored, repo-local)
2. Subagents: Emitted as .antigravity/agents/<plugin>__<agent>/agent.json
3. Skills: Emitted as .antigravity/skills/<plugin>__<skill>/SKILL.md (YAML frontmatter + body)
4. Commands: Emitted as .antigravity/workflows/<plugin>-<command>.md (as skill-style files for slash command support)
5. Global Installation: Symlink to ~/.gemini/antigravity-cli/ via make install-antigravity
6. Tool Mapping: Rewrite Claude Code CamelCase tools to Antigravity-native names (e.g. Read → view_file)
7. Model Mapping: Map Claude models to Gemini models (opus → gemini-2.5-pro, etc.)
8. Tests: Unit tests, structural validation, smoke tests

Out of scope

• Antigravity IDE/2.0 discovery (separate investigation)