This introduces `Op::UserTurn`, which makes it possible to override many
of the fields that were set when the `Session` was originally created
when creating a new conversation turn. This is one way we could support
changing things like `model` or `cwd` in the middle of the conversation,
though we may want to consider making each field optional, or
alternatively having a separate `Op` that mutates the `TurnContext`
associated with a `submission_loop()`.
---
[//]: # (BEGIN SAPLING FOOTER)
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* #2345
* __->__ #2329
* #2343
* #2340
* #2338
17aa394ae7