Assessment chats saved message by message

What changed

• Candidate chats with the assessment AI are now saved as each message is sent, so a reload brings the whole conversation back. Chats also keep working after a turn where the agent only edits files.
• Checkpoints for undoing agent edits now take much less browser storage, so each chat keeps a long history of them. Small style requests, such as changing a color, now edit the right file.
• The recruiter copilot can now delete assessments after the recruiter approves, and assessments it creates include the questions drafted in the chat. It also declines requests unrelated to recruiting.