#7083: 说明文档

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a new Chinese-language `GIT_SYNC_GUIDE.md` describing how to set up `origin`/`upstream` remotes for a fork, keep `main` in sync with upstream, and maintain a personal changes branch. The guide is straightforward and should be useful for contributors who maintain local tweaks, but a couple of the shell snippets are easy to mis-copy/paste (commented-out `git rebase` and merge-vs-rebase ordering) and there’s one environment-specific absolute path example that would be better as a generic placeholder. <h3>Confidence Score: 4/5</h3> - This PR is generally safe to merge; it only adds documentation, with a few small correctness/usability issues in command examples. - Changes are limited to a single new markdown doc. The main concerns are copy/paste correctness in shell snippets and one environment-specific path that could confuse readers; no code behavior changes. - GIT_SYNC_GUIDE.md (command snippet correctness and portability) <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->