ci(release): add workflow_dispatch prepare-release workflow

[luarss]

Summary

• Adds .github/workflows/prepare-release.yml with a workflow_dispatch trigger
• Dropdown input for bump_type: patch, minor, or major
• Mirrors the full logic from .claude/skills/release/SKILL.md as an automated CI workflow

What it does

1. Runs make test + make check before touching any files
2. Computes the new semver from pyproject.toml based on the selected bump type
3. Generates Keep-a-Changelog entries from conventional commits since the last tag (categorises feat: → Added, fix: → Fixed, chore:/ci:/build:/etc. → Changed, with PR links)
4. Updates pyproject.toml, server.json, README.md (URL pins), and CHANGELOG.md
5. Regenerates uv.lock
6. Commits as openroad-ci to a release/vX.Y.Z branch and opens a PR requesting review from @vvbandeira

Note: ROADMAP.md is intentionally not automated — that step requires human judgment about which future milestones are complete.

Test plan

• Trigger via Actions → Prepare Release → Run workflow with patch
• Verify the opened PR has correct version bumps across all files
• Verify CHANGELOG.md section is inserted correctly with commit categorisation
• Verify uv.lock is regenerated