clarify OpenCode native plugin npm requirement

[hopeatina]

Summary

• clarify that opencode.json native-plugin loading requires npm package 0.1.0-alpha.2 or newer
• point users to the direct peer command until npm publishing is fixed

Verification

• npm run type-check
• npm test
• npm run build
• node -e "import('./dist/index.js').then(async (m)=>{ if (typeof m.default !== 'function' || typeof m.startPeer !== 'function') process.exit(1); await import('./dist/peer.js'); console.log('opencode dist and peer imports ok') })"
• git diff --check