fix(agent_hub): verify visibility on incremental upload to existing repo

[mushenL]

Change Summary

Related issue number

增量上传到已有仓库时校验可见性，防止 private 静默泄露为 public

fix(agent_hub): verify visibility on incremental upload to existing repo

The private-visibility guard only ran when the repo was freshly created.
Incremental pushes into an already-existing repo skipped verification
entirely and could silently leak content to a public repo while reporting
"private". Verify before building the diff and before the up-to-date early
return, so the upload aborts before any byte is pushed or deleted.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Run pre-commit install and pre-commit run --all-files before git commit, and passed lint check.
• Documentation reflects the changes where applicable