Feature: per-project session color via settings.json

Problem

/color only works as a manual per-session command. There's no way to bind a color to a repository so it auto-applies when Claude Code starts.

Proposed Solution

Add a session.color setting to project-level .claude/settings.json:

{
  "session": {
    "color": "purple"
  }
}

When Claude Code starts in a directory with this setting, the session color is automatically applied — no manual /color needed.

Use Case

When working across multiple repos in parallel (e.g., in tmux splits), each session is immediately identifiable by color. This builds muscle memory and reduces context-switching friction.

This would pair naturally with the existing project-level settings.json and the /color command.