When toggling fast mode, the setting is now written to `settings.json`
under `agent.default_model.speed`, so new threads start with the same
speed. This follows the same pattern as `enable_thinking` and `effort`.
The `speed` field uses the existing `Speed` enum (`"fast"` /
`"standard"`) rather than a boolean, to leave room for future speed
tiers.
Example settings:
```json
{
"agent": {
"default_model": {
"provider": "zed.dev",
"model": "claude-sonnet-4",
"speed": "fast"
}
}
}
```
cc @benbrandt
Release Notes:
- N/A
---------
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
