Add default CLI response impl

## Summary
Seems like most of our CLI commands don't have anything specific to output, so we should provide a `Response::default()` impl that just serializes to an empty string.

## Proposal
- Impl `Default` for `Response`. e.g.
```rust
#[derive(Clone, Debug, Serialize)]
#[serde(untagged)] // <--------------------
pub enum Response {
String(String),
// ...
}

impl Default for Response {
fn default() -> Self {
Self::String("".to_string())
}
}
```
- Or make `Response` optional in the `Handler` trait, e.g.
```rust
fn handle(self) -> Result, Self::Error>;
```

Start by locating the CLI Response enum and Handler trait referenced in the proposal, then inspect how responses are serialized. Decide between a Default implementation and an optional Handler response, and verify that commands without specific output produce an empty string.

Written by the indexing model from the issue text.