Add ReactiveProperty release skill

[runceel]

Summary

Adds a contributor-facing skill for ReactiveProperty releases so agents follow the repository's current tag-driven NuGet publishing flow. This captures the release runbook in .agents/skills/ instead of treating it as a consumer-facing ReactiveProperty skill.

The skill covers release mode selection, version/tag validation, target commit preflight, local validation, tag push confirmation, GitHub Actions monitoring, and NuGet/GitHub Release verification. It also documents that version bump PRs are optional record-keeping only for the current workflow because package versions are derived from the v* tag during release.

Notes

The pre-release path requires -pre in the version because the current workflow uses that substring to mark GitHub Releases as prerelease.

Validation

• Checked skill metadata, line count, required release-mode ask_user guard, pre-release/stable mode coverage, error handling, output format, and post-run reflection.
• Confirmed referenced workflow, solution, version props file, and NuGet trusted publishing skill paths exist.