Auto-release to pub.dev on tagging.

This is peer issue for https://github.com/dart-lang/ecosystem/issues/418.

We need to work with the ecosystem maintainers (@mosuem and co) towards the fix to complete pub.dev release automation.

Review the linked ecosystem issue #418 to understand the context and coordination needed with dart-lang/ecosystem maintainers. Examine existing GitHub Actions workflows in the repository to see how releases are currently handled. The goal is to set up an automated release to pub.dev when a new tag is created, which likely involves configuring secrets, permissions, and a publishing script.

索引モデルが issue の本文から書いたものです。