[codex] Fix package module entry

[luuhongyii]

Summary

This fixes the package ESM entry in package.json.

The published package currently points module at dist/index.esm.js, but the package/build output contains dist/index.mjs instead. ESM consumers and tooling that prefer the module field can therefore resolve a file that is not present in the package.

Change

• Update module from dist/index.esm.js to dist/index.mjs.

Validation

• pnpm install --frozen-lockfile --ignore-scripts
• pnpm build
• npm pack --dry-run --json
• node -e "const p=require('./package.json'); console.log(JSON.stringify({main:p.main,module:p.module}))"
• git diff --check

I also ran pnpm test. It completed with 9/10 suites passing, 27/30 tests passing, and 33/36 snapshots passing. The remaining failures are three CSS snapshot diffs in src/test/global-style/global-style.test.ts showing output formatting/trailing-newline differences; I did not update snapshots because this PR only changes the package manifest entry.