SessionStart hook: support `sessionColor` output field to auto-set prompt bar color

Summary

Allow SessionStart hooks to set the session color programmatically via a new sessionColor field in hookSpecificOutput. This would let multi-repo or multi-environment workspaces automatically color-code sessions without requiring the user to manually run /color.

Motivation

/color is useful for distinguishing sessions at a glance, but today it's interactive-only. In a multi-repo workspace, the SessionStart hook already handles dynamic context injection (e.g., presenting a repo selection menu). The color assignment is a natural extension of that same flow — but there's no way to close the loop without manual user input.

Example use case: A workspace with 15+ repos uses a SessionStart hook to ask the user which repo to focus on. Based on the selection, the hook knows which color to apply — but can't act on it.

Proposed Change

Extend the hookSpecificOutput schema for SessionStart to accept an optional sessionColor field:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "...",
    "sessionColor": "green"
  }
}

Accepted values would mirror /color: red, blue, green, yellow, purple, orange, pink, cyan, default.

Behavior

• If sessionColor is set in hook output, apply it exactly as if the user had run /color <value>
• If the field is absent, no color change (current behavior preserved)
• If the value is invalid, silently ignore (don't error the session)