Why

Automate the GitHub Release creation process when a release pull request is merged into `main`. Currently, releasing requires manual steps after merging. This workflow should tag the release and publish a GitHub Release automatically.

Acceptance Criteria

• A new annotated git tag named `X.Y.Z` (e.g. `1.2.3`) is created at the merge commit on `main`
• The workflow triggers on `pull_request` closed events targeting the `main` branch
• The version is extracted from the PR head branch name, which must follow the pattern `release/X.Y.Z`
• A GitHub Release is created for the new tag