List custom agents as slash commands with a prefix.

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

Selecting a custom agent requires two steps, this cannot be represented in ACP protocol.

### Proposed solution

The proposed solution is to make the custom agent commands available as slash commands with a prefix to separate them. For example if the user has following custom agents

- review-tests
- java-review

Those custom agents will be able as slash commands through agent input and ACP available commands as

- agent::review-test
- agent::java-review

For CLI users this makes is easy to select custom agents with one selection. and the same can be easily make available through available commands in ACP model.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by tracing how custom agents are selected in the CLI and how ACP available commands are exposed. Verify that each custom agent appears as a prefixed slash command such as agent::review-tests and can be selected in one step, with the same commands available through ACP.

索引モデルが issue の本文から書いたものです。