Allow custom model aliases in /model command (e.g. low/medium/high)

Feature Request

Problem:
Switching between models for cost optimization requires knowing model names (haiku, sonnet, opus). It would be more intuitive to use semantic aliases like low, medium, high that map to models.

Proposed solution:
Allow users to configure custom aliases for the /model command in settings.json, e.g.:

{
  "modelAliases": {
    "low": "haiku",
    "medium": "sonnet",
    "high": "opus"
  }
}

Then /model high would switch to Opus, /model low to Haiku, etc.

Why this matters:

• Users optimizing token spend frequently switch models by task complexity
• Semantic names (low/medium/high) are easier to remember than model names that change over time
• Model names are versioned (e.g. claude-sonnet-4-6) — aliases abstract that away
• Could also support /effort-style UX where the aliases bundle model + effort level together

Current workaround:
Typing /model opus or /model sonnet manually. Custom slash commands (.claude/commands/) don't work for this since they're prompts, not CLI commands.