Document contribution and quality rules

Author: adhamcodes

CONTRIBUTING.md

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+# Contributing
+
+Thanks for helping improve this course.
+
+This repository is a textbook and reference implementation. Learner practice belongs in a separate disposable `git-github-lab` repository unless a contribution specifically improves the curriculum itself.
+
+## Before opening a change
+
+1. Read `SAFETY.md` if the change teaches recovery, reset, rebase, force operations, or history rewriting.
+2. Keep the course beginner-accessible. Prefer a precise mental model and a small reproducible exercise over adding more commands.
+3. Do not add claims of mastery that the learner cannot demonstrate.
+4. Do not add secrets, personal tokens, real credentials, or private repository data.
+5. Keep external resources supplementary; the core lesson must remain understandable without hunting through tutorials.
+
+## Required local check
+
+Run:
+
+```bash
+python scripts/validate_repo.py
+```
+
+The same validator runs on both Linux and Windows in GitHub Actions.
+
+## Pull request standard
+
+A useful pull request should explain:
+
+- what learner problem it solves
+- what behavior or understanding changes
+- how the change was tested
+- any safety implications
+
+For changes that add or modify a Git command exercise, include the expected observable repository state before and after the exercise.
+
+## Course design rule
+
+The learning loop is:
+
+> **LEARN → PREDICT → DO → INSPECT → BREAK/RECOVER → EXPLAIN → PROVE**
+
+Not every lesson needs every stage as a heading, but major capabilities should eventually require evidence rather than passive reading.