Persist /color setting across sessions via settings.json

Feature Request

Add support for persisting the prompt bar color preference across sessions.

Current Behavior

The /color command sets the prompt bar color for the current session only. Every time a new session starts, the color resets to default and the user must run /color <name> again.

Desired Behavior

Allow the color preference to be saved in settings.json (user or project scope) so it automatically applies at session start:

{
  "color": "orange"
}

Or alternatively, expose it as a SessionStart hook target so users can configure it themselves.

Motivation

Small but frequent friction — users who prefer a specific color must re-set it every session.

Available Colors

red, blue, green, yellow, purple, orange, pink, cyan, default