Usability feature: Add `shell` (or similar) as a dedicated slash(/) command

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

The CLI includes a special keyword `!` for executing shell commands, but this behavior is not discoverable or self documented. Unlike `/` commands, which are visible through autocomplete, `!` must be remembered by the user.

This creates a usability gap, especially for new or occasional users who may not know that shell execution exists or may forget how to access it.

### Proposed solution

Introduce a discoverable alternative such as a `/shell` command that behaves the same as `!`.

This command should appear in the `/` autocomplete list and clearly indicate that it allows shell execution. It could also include a short description or hint to explain its purpose.

### Example prompts or workflows

User types `/sh` and sees `/shell` in autocomplete
User selects `/shell` and enters shell execution mode, equivalent to using `!`
The CLI optionally shows a short hint such as “Run shell commands, same as `!`”

### Additional context

`/` commands are easy to find and use because they are discoverable through autocomplete. In contrast, the `!` keyword requires memorization, which makes it less accessible.

Adding a `/shell` command would improve discoverability, reduce the need to memorize special syntax, and create a more consistent user experience.

まず、既存のシェルキーワード `!` の CLI エントリーポイントと、`/` コマンドの自動補完実装を見つけます。シェル実行に関する短いヒントを備えた、見つけやすい `/shell` コマンドを追加し、`!` と同等の動作を維持します。`/sh` で候補として表示され、選択するとシェル実行モードに入れば完了です。

索引モデルが issue の本文から書いたものです。