Sampler

[granawkins]

Two big fixes needed:

• When there are no edits made and you run /sample, the diff_active shows up as diff_edit instead.
• For old conversation messages that include file edits, we're currently ignoring them, but we should convert to diff and include.
• Message history is backwards
• A prototype field for when we add/remove context, or other edits from an existing sample. There maybe overlap with some benchmark functionality.
• Optional labels (list of strings), like 'extra-context'