[Feature Request] Add setting to randomize accent color on session start

Feature Request

Add a settings.json option to automatically randomize (or set) the session accent color when a new session starts.

Motivation

The /color command lets users set the prompt bar color during a session, but there's no way to automate this. Users who want a fresh random color each session have to manually run /color every time. Since /color is an internal UI command, it can't be triggered via SessionStart hooks either — hooks can only run shell commands, LLM prompts, or agents.

Proposed Solution

A color setting in settings.json:

{
  "color": "random"
}

Accepted values:

• "random" — pick a random color on each session start (from the full color space, not just named colors)
• A named color ("red", "blue", "cyan", etc.) — always use that color
• A hex color ("#1abc9c", "#e74c3c", etc.) — allow arbitrary colors beyond the preset names
• Absent/"default" — current behavior (no color override)

Hex color support

The current /color command only accepts a small set of named colors. Supporting hex values (both in the setting and in /color itself) would allow much better personalization and true randomization across the full color spectrum rather than cycling through ~8 presets.

Alternatives Considered

• SessionStart hook: Hooks can't invoke slash commands, so this isn't possible today.
• CLI flag (--color random): Would work but is less ergonomic than a persistent setting.
• Shell alias/wrapper: Fragile and doesn't integrate with the IDE extensions or web app.