Add Gemini plugin output

[chubes4]

Summary

• Add a generated Gemini output alongside the existing Codex and Claude Code packages.
• Package shared WordPress skills, GEMINI.md, and Gemini CLI MCP settings for WordPress Studio and telemetry.
• Extend build, verification, and top-level docs to cover the Gemini output.

Testing

• pnpm build
• pnpm verify

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the Gemini packaging implementation, generated docs/instructions, and ran verification for Chris to review.