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.

Start by tracing how ~/.coppier/settings.json is loaded and how existing slash commands such as /model and /rubber-duck are dispatched. Done means users can define aliases there and invoke them to select the mapped model or run the mapped prompt, including the examples described in the issue.

Rédigé par le modèle d'indexation à partir du texte de l'issue.