feat: add Google Gemini / Antigravity Agent support

[FACUTUCCI10]

This PR adds first-class support for the Google Gemini CLI / Antigravity Agent to the GStack repository. It enables Gemini-based agents to load and run GStack's virtual engineering team skills natively and in isolation, avoiding path collisions or environment conflicts with Claude or Codex.

Key Changes

New Host Configuration (hosts/gemini.ts): Defines path rewrites (.claude/ -> .gemini/), suppressed resolvers (GBRAIN_CONTEXT_LOAD, GBRAIN_SAVE_RESULTS to bypass standard local gbrain hooks), and allowlists for frontmatter fields (name, description).

Host Registration (hosts/index.ts)**: Registered gemini within ALL_HOST_CONFIGS.

Installer Update (setup): Integrated Gemini path resolution, flag parsing (--host gemini / --host auto), and directory mapping during setup.

Windows Path Normalization (scripts/gen-skill-docs.ts)**: Normalizes generated relative paths to use Unix-style slashes (/), resolving dryness/freshness test suite assertions on Windows.

Test Updates (test/host-config.test.ts)**: Incremented default host count expectations and registered export checks to match the new host.

Documentation (README.md & Gitignore)**: Documented the new --host gemini option in the supported agents table and added .gemini/ output paths to .gitignore.

Verification Results
Ran bun run scripts/host-config-export.ts validate and verified all 11 configs are valid.
Ran bun test test/gen-skill-docs.test.ts on Windows: 408 tests passed / 0 failed.
Verified manual generation output via bun run scripts/gen-skill-docs.ts --host gemini.

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