chore: align use package surface

[aose-yuu]

Summary

• README を代表例中心の構成に更新
• package exports と prepack を他 repo と揃える
• release workflow の説明を CONTRIBUTING に追加

Tests

• pnpm install
• pnpm -s test
• pnpm -s typecheck
• pnpm -s build
• pnpm -s format
• rg -n '\t' README.md\n- root ESM import / CJS require check\n- npm pack --dry-run --json

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 522279c06e

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Restore the package.json subpath export

When consumers or tooling import or require @sigrea/use/package.json to read package metadata, this export map now rejects that previously public subpath with ERR_PACKAGE_PATH_NOT_EXPORTED because the ./package.json entry was removed. Since npm still includes package.json in the published tarball, adding the subpath back preserves the existing package API without changing the runtime bundle.

Useful? React with 👍 / 👎.