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.

首先定位現有 shell 關鍵字 `!` 的 CLI 入口點，以及 `/` 命令自動補全的實作。新增一個容易找到的 `/shell` 命令，並提供簡短的 shell 執行提示，同時維持與 `!` 等效的行為；當 `/sh` 會建議該命令且選取它會進入 shell 執行模式時，即表示完成。

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