ci: Add tag-triggered release workflow

[cuiko]

Summary

• Add .github/workflows/release.yml that fires on v* tag pushes and publishes a GitHub Release with the bundled DMG.
• Release notes auto-generated from PR titles merged since the previous tag (gh release create --generate-notes).
• Requires only contents: write permission; uses the default GITHUB_TOKEN, no extra secrets.

After merge the release ritual collapses to:

# bump CFBundleVersion + CFBundleShortVersionString in Info.plist
git add Info.plist
git commit -m "chore: Bump version to x.y.z"
git tag vx.y.z
git push origin main --tags

CI on macos-14 picks up the tag, runs make dist, and creates the Release. No local make dist or gh release create step needed anymore.

Test plan

• Workflow file is syntactically valid (visible green check in Actions tab once on main)
• Next real release (likely v1.3.0 after the SPM refactor in ref: Reorganise into Swift Package Manager layout #2 merges) produces a DMG-bearing GitHub Release automatically without manual gh invocation