refactor(vite-plugin)!: extract the Vite plugin and invert the core dependency

[harijoe]

Summary

• Moves the Vite plugin out of skybridge into its own package, @skybridge/vite-plugin.
• Breaking: skybridge/vite is removed; import skybridge from @skybridge/vite-plugin instead.
• Moves view discovery into core, exposed as skybridge/views, so the dependency runs core → plugin and not both ways.
• Mechanical import churn across examples, templates and e2e; no behaviour change.

Architecture Notes

• Dependency direction — core's CLI needs scanViewsSync, and the plugin needs the same discovery. Core now owns it and the plugin depends on core, which is why prepare and the publish workflow build core first.