Add support for slash commands in `copilot --acp`

### Describe the bug

Copilot ACP doesn't provide `available_commands_update`, so ACP frontend is unable to use slash commands.

### Affected version

0.0.388

### Steps to reproduce the behavior

(in zed)

1. add Custom agent like
```jsonc
"agent_servers": {
// ...
"Copilot Cli": {
"type": "custom",
"command": "copilot",
"args": ["--acp"],
}
},
```
2. try `/model`
3. error with `Available commands: none`

### Expected behavior

_No response_

### Additional context

- Zed 0.219.5
- zed impl: https://github.com/zed-industries/claude-code-acp/blob/36ea238d76490c60a68150e926095ef5d32ea759/src/acp-agent.ts#L796-L805
- acp docs: https://agentclientprotocol.com/protocol/slash-commands