feat(install): add Antigravity skill installation support (#87)

[tanvir70]

Add Antigravity skill installation support to rgctl install --skill.

• Add Antigravity variant to SkillHost (--host antigravity)
• Add Antigravity variant to InstallWriteHost with lowercase serialization
• Map antigravity in host_compat
• Include antigravity in DEFAULT_INSTALL_AGENTS alongside cursor, claude, codex, and agents
• Add regression coverage in tests/install_skill.rs and tests/cli_output/install.rs
• Update CLI, JSON API, installation, agent guides, and website documentation

Description

Brief description of what this PR does.

Fixes #(issue number)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement
• Documentation update
• Language support (new language plugin)
• Refactoring (no functional changes)

Changes Made

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• Change 1
• Change 2
• Change 3

Testing

How has this been tested?

• Unit tests added/updated
• Integration tests added/updated
• Tested with cargo test
• Manual testing performed

Test environment:

• OS: (e.g., macOS, Linux, Windows)
• Rust version: (output of rustc --version)

Checklist

• My code follows the style guidelines (ran cargo fmt)
• I have performed a self-review of my code
• I have commented my code where necessary
• I have added tests that prove my fix/feature works
• All new and existing tests pass (cargo test)
• No clippy warnings (cargo clippy --all-targets -- -D warnings)
• Documentation updated (if applicable)
• CONTRIBUTING.md guidelines followed
• All commits are signed and include DCO sign-off (Signed-off-by:) — see Developer Certificate of Origin and contributor checklist §6

Performance Impact

If this PR affects performance:

• Benchmarks run (attach results if significant)
• No measurable performance regression
• Performance improvement measured: X%

Breaking Changes

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What breaks:

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Migration path:

• How should users update their code/configuration?

Screenshots (if applicable)

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Additional Notes

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