Allow custom slash commands to switch default model

Feature Request

Allow custom slash commands to execute built-in commands like /model haiku or provide a mechanism to change the session's default model programmatically.

Use case: Creating shortcut commands like /h that can both:

1. Switch default model to haiku when run without args
2. Run a one-off query with haiku when given args

Current behavior: The model: frontmatter in slash commands only affects that command's execution model, not the session default. Editing settings.json doesn't take effect until restart.

Desired behavior: Either:

• Live-reload settings.json when modified
• Expose a mechanism for slash commands to call built-in commands
• Add a switch_model: frontmatter option that persists the change