Allow skills to invoke built-in slash commands (e.g. /color)

Feature Request

Skills currently cannot programmatically invoke built-in slash commands like /color, /theme, etc. This limits what skills can automate.

Use Case

I built a /focus skill that sets up a project-focused session — changes working directory, loads project-specific context, and sets context boundaries. The last piece is setting the prompt bar color to visually distinguish parallel sessions (e.g., cyan for one project, yellow for another).

Right now the skill has to tell the user "type /color cyan" as a manual step, which breaks the flow of an otherwise fully automated setup.

Proposal

Allow skills to invoke built-in slash commands, or at minimum expose prompt bar color as a setting in settings.json so it can be set programmatically via file edit.

Either approach would work:

1. A color key in settings.json (e.g., "color": "cyan")
2. An API/tool that skills can use to invoke built-in commands

Context

This would benefit any skill that wants to configure session-level UI state as part of its setup flow.