add native OpenCode plugin entrypoint

[hopeatina]

Summary

• add a native OpenCode plugin export/default export that starts the existing OrgX peer on server.connected
• keep CLI/programmatic startPeer() support while lazy-loading the peer runtime from the package entry
• document opencode.json installation and add OpenCode plugin keyword
• add explicit .js internal ESM imports so built Node imports work

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

Notes

• Depends on merged SDK ESM fix: fix node esm exports orgx-gateway-sdk#4