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.

首先追蹤 ~/.coppier/settings.json 的載入方式，以及 /model 和 /rubber-duck 等現有斜線指令的分派方式。完成的標準是使用者可以在其中定義別名並呼叫它們，以選取對應的模型或執行對應的提示詞，包括 issue 中描述的範例。

由索引模型根據 Issue 內容生成。