docs(ecosystem): add oh-my-opencode to plugins list

[code-yeongyu]

Summary

Adds oh-my-opencode to the Plugins section of the ecosystem page.

Details

oh-my-opencode is a comprehensive plugin distribution, every good thing is curated, after my personal experimentation

It provides:

• Background agents & multi-model orchestration (Oracle, Librarian, etc.)
• Pre-built tools including LSP clients and AST-Grep
• A compatibility layer for Claude Code extensions

Repository: https://github.com/code-yeongyu/oh-my-opencode