Configure custom aliases

### Describe the feature or problem you'd like to solve

Allow to create aliases and custom /commands

### Proposed solution

Let users define their own slash command aliases, mapped to specific models and prompts.
Why it helps:
- Faster model switching, less typing
- Consistent workflows across tasks
- Feels natural, like shell or git aliase

### Example prompts or workflows

For example, modifying `~/.coppier/settings.json` to set
- `/m1` as an alias for `/model claude-opus-4.8`
- `/m2` for `/model gpt-5-mini`
- `/rd` for `/rubber-duck`
- `/docs` for "Rewrite this README clearly"

### Additional context

This fits how aliases are already used in git and the shell. Small feature, big usability win.