Fixes bd-130
Fixed all CI failures that caused the original revert:
- Windows: Use filepath.ToSlash() for git paths (git always uses forward slashes)
- Nix: Skip test when git not available (NIX_BUILD_TOP env check)
- JSON parsing: Increased scanner buffer to 64MB for large issue descriptions
- Cross-platform: Added normalizeGitPath() helper for path comparisons
Also restored beads.jsonl > issues.jsonl precedence in checkGitForIssues().
All tests pass locally.
Amp-Thread-ID: https://ampcode.com/threads/T-a560d9a0-29b9-4c46-aa90-813758d2553c
Co-authored-by: Amp <amp@ampcode.com>
0caf423021