Add GitHub Action diff mode

[Conalh]

Summary

• Add scopetrail diff --repo <repo> --base <ref> --head <ref> by materializing git snapshots into temporary directories.
• Add a composite GitHub Action that writes a ScopeTrail Markdown report to the GitHub Actions step summary and supports advisory/failing thresholds.
• Add a self-dogfooding PR workflow plus tests for git refs, action metadata, and workflow wiring.

Validation

• npm run build
• npm test (8 tests, 0 failures)
• node dist/index.js diff --repo . --base origin/main --head HEAD --format markdown