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>