docs(web): add opencode-antigravity-img plugin to ecosystem

[ominiverdi]

Summary

Adds opencode-antigravity-img to the plugins list in the ecosystem documentation.

This plugin enables image generation using the Gemini 3 Pro Image model via the Antigravity/CloudCode API. It's a companion to opencode-antigravity-auth (already listed).

Links:

• GitHub: https://github.com/ominiverdi/opencode-antigravity-img
• npm: https://www.npmjs.com/package/opencode-antigravity-img

Features:

• generate_image tool - Generate images from text prompts
• image_quota tool - Check remaining quota for image generation