Fall back to old key when loading agent settings (#30001)

This PR updates #29943 to fall back to loading agent panel settings from
the old `assistant` key if the `agent` key is not present. Edits to
these settings will also target `assistant` in this situation instead of
`agent` as before.

Release Notes:

- Agent Beta: Fixed a regression that caused the agent panel not to
load, or buttons in the agent panel not to work.

Cargo.lock

@@ -608,6 +608,7 @@ dependencies = [
  "paths",
  "schemars",
  "serde",
+ "serde_json",
  "serde_json_lenient",
  "settings",
  "workspace-hack",