Allow persisting /color setting per-project in settings.json

Feature Request

Allow the /color session color to be persisted in settings files (e.g., settings.json or settings.local.json) so it automatically applies when opening Claude Code in a specific project.

Use Case

When working across multiple repos, it's helpful to visually distinguish sessions by color. Currently /color blue must be run manually every time a new session starts. A persistent setting would make this automatic.

Proposed Solution

Add a color field to the settings schema:

{
  "color": "blue"
}

This could be set in any settings scope:

• ~/.claude/settings.json (global default)
• .claude/settings.json (project-wide)
• .claude/settings.local.json (personal per-project)

Alternatively, make /color work from a SessionStart hook.

Current Behavior

• /color blue works per-session but resets on next session
• No color field exists in the settings schema
• SessionStart hooks cannot invoke internal CLI commands like /color