ci: sync main and docs branches to Gitee

[yanglbme]

Summary

Add sync-gitee.yml so a push to main force-pushes the same branch to Gitee, without holding the deploy-site lock.

• Mirrors only main or docs (never Gitee pages)
• docs pushes are handled by a follow-up PR that dispatches this workflow

Type

• Other

Checklist

For new or updated solutions:

• README.md / README_EN.md follow solution/template.md
• If a Thinking block is added, it sits after the method heading, is non-empty, and matches the code in the repo (naive idea vs constraints, bottleneck, key observation, why this method). Thinking is optional.
• All languages implement the same algorithm; unrelated Solution.* files and code tabs are unchanged
• Code is formatted as described in AGENTS.md

Test plan

• Merge this PR before the docs trigger PR
• Push (or re-run) on main and confirm Gitee main moves to the same SHA
• Confirm a running sync-gitee does not queue deploy.yml

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