Format streamed edits on save

[rtfeldman]

Re-enables format on save for agent changes (when the user has that enabled in settings), except differently from before:

• Now we do the format-on-save in the separate buffer the edit tool uses, before the diff
• This means it never triggers separate staleness
• It has the downside that edits are now blocked on the formatter completing, but that's true of saving in general.

Release Notes:

• N/A