docs: rewrite chapter 7 as a recipe-based playbook

Replace sequential exercises with ~50 quick-reference recipes
grouped by topic: undoing changes, branching, merging, rebasing,
remote operations, cherry-picking, stashing, tagging, submodules,
debugging, and configuration. Each recipe shows the problem and
the exact commands to solve it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj