feat: Add Google Antigravity CLI harness support

### Description Implement support for the new **`antigravity`** harness (representing the successor to Gemini CLI, the Antigravity CLI) in the multi-harness agentic plugin marketplace. ### Requirements 1. **Output Directory**: `.antigravity/` (gitignored, repo-local) 2. **Subagents Format**: Emitted as `.antigravity/agents/{agent_id}/agent.json` containing the `customAgentSpec` structure. 3. **Skills Format**: Emitted as `.antigravity/skills/{skill_id}/SKILL.md` (markdown file wrapped in YAML frontmatter). 4. **Command-as-Skill Format**: Emitted as `.antigravity/skills/{plugin_name}-{command_name}/SKILL.md` (with `user-invocable: true` and `disable-model-invocation: true`). 5. **Global Installation**: Safely symlink `.antigravity/` to the global path `~/.gemini/antigravity-cli/` via `make install-antigravity`. 6. **Tool Mapping**: Rewrite Claude Code CamelCase tool names to Antigravity-native names (e.g. `Read` -> `view_file`, `Bash` -> `run_command`, etc.). 7. **Model Mapping**: Map Claude models (`opus`, `sonnet`, `haiku`) to Gemini models (`gemini-2.5-pro`, `gemini-2.5-flash`). 8. **Tests & Validation**: Structural validation in `validate_generated.py`, doc gardening in `doc_gardener.py`, and comprehensive unit tests.