docs: add safety notes for optimization runs

[jiezeng2004-design]

Summary

• add quick-start safety notes before running optimization
• remind users to commit or stash local skill edits first
• remind users to review checkpoint diffs for risky workflows

Testing

• not run; documentation-only change

[jiezeng2004-design]

Hi @alchaincyf — gentle ping on this. The safety notes should help users avoid accidentally running optimization in interactive mode without confirmation. Happy to rephrase or relocate if you'd like a different approach.

[jiezeng2004-design]

Hi @alchaincyf, gentle follow-up on this docs-only safety note. It is meant to help users avoid running optimization flows without clear confirmation. Happy to rephrase or move it if another section fits better.