Update API reference copy to nearest .github in repo

[xavierjohn]

Refactor Trellis.ApiReference.targets to copy API reference markdown files to the nearest existing .github directory up to the .git repo root, rather than always using the repo root. Falls back to creating .github at the repo root if none exists. Improves flexibility for multi-project repos and ensures files are only copied within the intended repository boundary. Updates comments and variable names for clarity.

[github-actions]

Test Results

330 tests  ±0   330 ✅ ±0   51s ⏱️ -2s
  4 suites ±0     0 💤 ±0 
  4 files   ±0     0 ❌ ±0 

Results for commit ce9a340. ± Comparison against base commit fc0c1fc.

[Copilot]

Pull request overview

This PR refactors the packaged MSBuild targets used for syncing Trellis API reference markdown into a repository’s .github/ folder, so the destination is the nearest existing .github directory when walking upward from the project directory, bounded by the nearest .git repo root (with a fallback to creating .github/ at the repo root). This improves behavior in multi-project / nested-repo layouts while preventing copies from escaping the repository boundary.

Changes:

• Change destination resolution to prefer the nearest existing .github/ directory between the project and the .git repo root.
• Add explicit repo-boundary guarding logic to avoid selecting a .github/ directory above the discovered .git root.
• Rename/adjust internal properties and comments for clarity (e.g., _TrellisOutputDir, _TrellisNearestGitHub, expanded explanation of the unrolled walk).