Allow persistent prompt bar color via setting, flag, or hook output

Summary

/color <name> changes the prompt bar color for the current session only. There's no way to set it automatically on launch.

Motivation

I use a SessionStart hook that assigns each session a random color + planet name (e.g. "magenta Jupiter") shown in my status line. I'd like the prompt bar to match so the session's color identity is visible in both places without typing /color magenta manually every launch.

Proposed options (any one would solve this)

1. A color field in settings.json (user or project scope) read at startup.
2. A --color <name> CLI flag.
3. A hookSpecificOutput.color field that SessionStart hooks can emit.
4. An env var like CLAUDE_PROMPT_COLOR honored at launch.

Current workarounds

None clean — /color isn't writable from hooks, Claude can't invoke slash commands, and there's no persistent field. Best current option is glancing at the status line and typing /color <name> each session.

Environment

Claude Code on Windows 11, shell: bash.