Add `build` field to plugin.json so stdio servers can be automatically
compiled to native binaries during install, eliminating runtime deps.
- Add BuildConfig struct to Manifest (command + output fields)
- Add BuildPlugin/runBuild in loader with output verification
- Auto-trigger build in InstallFromDir and InstallFromGit (rollback on failure)
- Add `dws plugin build <dir>` command for manual builds
- Update `plugin create` scaffold to include build template