bump OpenCode plugin alpha release

[hopeatina]

Summary

• bump package and plugin manifest to 0.1.0-alpha.3 so the native OpenCode plugin entrypoint can be released from current main
• keep the package and heartbeat manifest versions aligned

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') })"
• npm pack --dry-run --json
• git diff --check

Release note

After merge, publish GitHub prerelease v0.1.0-alpha.3. If npm trusted publishing is correctly configured, Actions should publish the package with the alpha dist-tag; otherwise the run will continue proving the external npm auth blocker tracked in issue #7.