Prepare for 0.2.1 release

[Aaronontheweb]

Summary

• Bump version to 0.2.1 in Directory.Build.props
• Add 0.2.1 release notes to RELEASE_NOTES.md

Release Notes

New Features

• Add resource file upload support to POST /skills — skills can now include arbitrary resource files (e.g. references/guide.md, scripts/setup.sh) per the AgentSkills.io spec (Wire upload endpoint to accept reference files #52)

Improvements

• Resource upload endpoint now supports any subdirectory per the AgentSkills.io spec — previously restricted to references/ only; all subdirectories are now accessible with path traversal protection (Clean up resource upload endpoint #53)
• Fix stream leak on validation failure in resource upload path (Clean up resource upload endpoint #53)

Test plan

• CI passes (Test, NuGet Pack, Slopwatch, Docker Build)
• Version bump is correct in Directory.Build.props
• Release notes are present and accurate