/auto (#16696)

Add `/auto` behind a feature flag that's disabled for now, even for
staff.

We've decided on a different design for context inference, but there are
parts of /auto that will be useful for that, so we want them in the code
base even if they're unused for now.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/assistant/src/assistant_settings.rs

@@ -519,6 +519,7 @@ impl Settings for AssistantSettings {
                 &mut settings.default_model,
                 value.default_model.map(Into::into),
             );
+            // merge(&mut settings.infer_context, value.infer_context); TODO re-enable this once we ship context inference
         }
 
         Ok(settings)