[Feature Request] Persist prompt color across sessions via settings.json

Bug Description
Feature request: Persist /color across sessions via settings.json

Currently /color is session-only with no way to persist it. It would be great to have a promptColor setting in .claude/settings.json
(both user-level and project-level), so different projects can automatically get their own color.

Use case: I work across multiple Obsidian vaults and project directories. I already use different terminal themes per project, but having
the Claude prompt bar color persist per project would make session identification even easier.

Suggested implementation:

• Add promptColor to settings.json schema
• Project-level settings.json overrides user-level
• /color command updates the setting when used (optional)

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.75
• Feedback ID: 1b07a720-ed7c-4617-ae8a-ae84f9602c9d