Rename an expert or replace its instructions

Update one of the user's experts: its name, its instructions, or both. instructions replaces the WHOLE body: read it with get_expert first and send the whole text with the change in it, a line that contradicts an existing one replacing that line rather than sitting beside it. A rename to a name the user already has is refused. The previous body stays in the expert's version history (list_versions, restore_version). Use it when the user says an expert missed something, should work differently, or should be called something else.