rsbuild-init-scaffold.md

arkenv	minor

feat(cli): Rsbuild template detection and scaffolding in arkenv init

Added first-class support for Rsbuild in arkenv init:

• Detects @rsbuild/core in project dependencies or rsbuild.config.* files.
• Adds @arkenv/rsbuild-plugin to project dependencies during initialization.
• Bootstraps rsbuild.config.* files by injecting arkenvRsbuildPlugin().
• Sets default client prefix to PUBLIC_.