Add SKILL.md and auto-copy to consuming repos

Added SKILL.md with comprehensive StructId documentation. Updated StructId.Package.msbuildproj to package SKILL.md under skills/structid/. Introduced a CopyStructIdSkill MSBuild target in StructId.targets to auto-copy SKILL.md to .github/skills/structid/ in consuming repos, improving downstream documentation discoverability.

Author: kzu