Fallback to SolutionDir when git root is unavailable for skill copy

[Copilot]

When SourceLink cannot resolve a git root, CopyStructIdSkill silently skips copying SKILL.md to .github/skills/structid/. This leaves consumers without the skill file in non-git or SourceLink-unconfigured scenarios.

Changes

• StructId.targets: After attempting to resolve _StructIdSkillRepoRoot from the git SourceRoot, falls back to $(SolutionDir) if the result is still empty (and SolutionDir is defined/not *Undefined*). The copy is only skipped when neither source can provide a base directory.

<PropertyGroup>
  <_StructIdSkillRepoRoot>@(_StructIdSkillSourceRoot)</_StructIdSkillRepoRoot>
  <_StructIdSkillRepoRoot Condition="'$(_StructIdSkillRepoRoot)' == '' and '$(SolutionDir)' != '' and '$(SolutionDir)' != '*Undefined*'">$(SolutionDir)</_StructIdSkillRepoRoot>
</PropertyGroup>

[kzu]

🧪 Details on Ubuntu 24.04.4 LTS

🧪 Details on macOS Unix 15.7.4

🧪 Details on Microsoft Windows 10.0.26100

from retest v1.1.0 on .NET 10.0.6 with 💜 by @devlooped