ci: trigger crates.io publish on GitHub Release instead of tag push

[tbille]

Summary

• Changes the release workflow trigger from tag push (v*) to GitHub Release publish event
• Removes the github-release job (no longer needed since the release already exists when the workflow runs)
• Updates version tag extraction to use github.event.release.tag_name instead of GITHUB_REF

Motivation

Publishing to crates.io should be triggered by creating a GitHub Release, not by pushing a tag directly. This makes the release the single source of truth and gives better control over the publish process (e.g., draft releases won't trigger a publish).

Release workflow after this change

1. Bump the version in Cargo.toml and merge to main
2. Create a GitHub Release (via UI or gh release create v0.1.0)
3. The workflow triggers automatically, verifies the version matches the tag, runs tests, and publishes to crates.io

Prerequisites for first publish

The CARGO_REGISTRY_TOKEN secret must be configured in the repository settings with a valid crates.io API token.