Support random prompt color on startup

Feature request

Add support for randomizing the prompt bar color on startup, either via:

• A promptColor: "random" setting in settings.json
• A --color CLI flag (e.g. claude --color random)
• Making /color triggerable from a SessionStart hook

Currently /color is an interactive-only slash command with no way to automate it. A config-level setting would let users get a fresh color each session without manual input.

Use case

Variety! A different color each time makes it easy to visually distinguish sessions.