Rebuild Git & GitHub course as an executable self-study course

[adhamcodes]

Final owner-reviewed rebuild

This rebuild preserves the course's strong core and closes the remaining artifact-quality gaps.

Shipped

• real challenge sheets for every module; empty exercise scaffolding removed
• self-contained GitHub Actions lab with a runnable pass/fail/repair cycle
• current GitHub Actions majors and Python 3.12/3.14 validation
• Linux + Windows repository quality gates
• structural/local-link/empty-file/placeholder/secret-pattern checks
• real disposable Git behavior smoke test covering merge conflict and reflog recovery
• safer history-rewrite guidance including --force-with-lease
• CONTRIBUTING rules and MIT license
• learner path wired as lesson → challenge → cumulative gate → capstone

Independent verification

The exact PR head has a green Course quality gates run. The rebuild is ahead of main with no divergence from the audited baseline.

Safety

The original state remains preserved at baseline/pre-100-rebuild-2026-09-04.

Owner review is complete. This PR is ready for the final squash merge.