Add versioned GitHub releases (tag-driven, hance-style)

This repo has no releases, so there is no way to pin or reference a known-good version of the skill.

Adopt the release model used by Orva-Studio/hance:

- The git tag is the single source of truth for the version. Pushing a `v*` tag (or a manual `workflow_dispatch` with a tag input) triggers a `release.yml` workflow.
- The workflow derives the version from the tag (`${TAG#v}`) rather than a version field committed in the repo, so nothing needs bumping in source.
- The workflow creates the GitHub release with generated notes attached to the tag.

Scope for this repo is much smaller than hance's (no binaries to build or sign): the workflow only needs to create the release, optionally attaching a tarball of `skills/last30flames/` so a specific skill version can be fetched directly.

Out of scope: manual CHANGELOG.md maintenance (auto-generated notes only), publishing to any registry.

Start by inspecting the repository's existing GitHub Actions configuration and the skills/last30flames/ directory, then use the release.yml workflow described in the issue as the entry point. Done means pushing a v* tag, or using workflow_dispatch with a tag input, creates a GitHub release with generated notes and optionally attaches a tarball of the skill.

Written by the indexing model from the issue text.