ci: automate idempotent GitHub Releases #1

Problem

Publishing a GitHub Release is currently a manual step, which can let the package version, Git tag, and release metadata drift apart.

Proposed solution

Add a GitHub Actions workflow that runs after tests on pushes to main, derives the release tag from package.json, creates an annotated tag at the pushed commit only when absent, and creates a generated-notes GitHub Release only when absent.

Acceptance criteria

• The workflow runs the existing test suite before release operations.
• The tag is derived as v<package.json version>.
• A missing tag is created as an annotated tag at the pushed main commit.
• An existing tag is never moved or recreated.
• A missing GitHub Release is created with generated notes.
• An existing GitHub Release is left unchanged.
• Re-running the workflow is idempotent.
• The README documents installation and release behavior.

Scope and non-goals

Scope is limited to the release workflow and its README documentation. It does not publish to npm, bump versions, retag existing versions, or create releases from non-main branches.