Agents window controls and paired company questions

What changed

• Chats in the Agents window can now be pinned, renamed, forked from any reply, and copied as a transcript. While you scroll back, your latest prompt stays pinned at the top.
• A pending edit can be kept or rejected one file at a time. Reverting to an earlier prompt now restores every file, so the preview matches the chat again.
• Every company-inspired systems design question now has an assessment version that builds the same system with one harder requirement. Recruiters can use that version in a real assessment, so candidates cannot have practiced the exact question beforehand.