docs: add link to OpenCode port (opencode-brain)

[deiviuds]

I've created an OpenCode-compatible version of claude-brain called opencode-brain. It maintains 100% file format compatibility with the .claude/mind.mv2 file, so users can use both tools interchangeably on the same project.

What is opencode-brain?

opencode-brain is a port of claude-brain for OpenCode users, providing the same photographic memory functionality.

Key Features

• 100% file format compatible - shares the same .claude/mind.mv2 file
• Works with both tools - users can switch between Claude Code and OpenCode seamlessly
• Available on npm - easy installation with npm install opencode-brain
• Same memvid SDK - uses the same underlying technology
• Zero lock-in - users aren't forced to choose between Claude Code and OpenCode

Links

• npm: https://www.npmjs.com/package/opencode-brain
• GitHub: https://github.com/deiviuds/opencode-brain
• Documentation: See README for full details

Changes in this PR

Added an "OpenCode Support" section to the README after the CLI section, linking to opencode-brain for OpenCode users who want the same memory persistence functionality.

This helps expand the reach of claude-brain's memory format to the OpenCode community while maintaining full compatibility.