Fix SiteX package publishing setup

[silveltman]

Summary

• publish compiled SiteX package entrypoints instead of raw TypeScript from node_modules
• add Changesets release and CI workflows with npm trusted publishing support
• narrow published dependencies and update install docs for pnpm esbuild approval

Verification

• pnpm check
• pnpm build:package
• pnpm build
• packed tarball installed in a fresh temp consumer project
• consumer pnpm typecheck
• consumer pnpm build