Add `color` setting in settings.json to persist default session color

The /color command lets you set the session UI color, but the setting is lost when the session ends. There's no way to persist a preferred color across sessions.

Requested change

Add a color key to the settings schema so users can set a default:

{
  "color": "cyan"
}

This would apply the color automatically at session start, equivalent to running /color cyan each time.

Workaround attempted

A SessionStart hook could theoretically automate this, but /color is a slash command and can't be invoked from a shell hook.

Available colors

From current /color validation: red, blue, green, yellow, purple, orange, pink, cyan, default