[Bug] documentation instructions for update strategy "git remote merge" are wrong

## Description The documentation in [UPGRADING.md](https://github.com/Donchitos/Claude-Code-Game-Studios/blob/984023ddac0d5e27624f2baacde6105e45de375f/UPGRADING.md#L43) outlines the workflow to update the templates when you forked the repository to build your game (common strategy). However, it wrongly states to use "git merge --allow-unrelated-histories" which creates merge conflicts for all files since there is no common history for git to assess, so it flags everything from the template as a merge conflict. The correct way is to NOT use merge, but a selective checkout like ``` git checkout template/main -- .claude ``` This gives you a clean changeset of upstream template changes which you can commit in a single chore commit, and continue with fresh templates in your next session. ## Environment - **OS**: macOS 26.5 "Tahoe" - **Shell**: zsh - **Claude Code version**: 2.1.150 - **Node.js version**: n/a - **jq installed?**: Yes - **Python installed?**: Yes (3.12) ## Affected Component - [ ] Agent (which one?): - [ ] Skill (which one?): - [ ] Hook (which one?): - [ ] Rule (which one?): - [ ] Template - [X] Documentation - [ ] Other: ## Additional Context Any other context — screenshots, terminal output, or the session log if relevant.