swarm-controller: refuse an inference model name that starts with "-"

`check_model` accepted a value like `--dangerously-skip-permissions`,
and the stored model ends up in the agent's `claude` argv. A PUT whose
`model`, or any `subagent_models` entry (same validator), starts with
`-` is now a 400 with the same problem+json shape as the other model
checks. A dash inside a name (`claude-opus-4-8`) is still accepted.

Raised by argus on #5084.

Refs #5034

swarm-controller/src/main.rs

@@ -5544,17 +5544,16 @@ mod tests {
                 "DELETE {name}"
             );
         }
-        assert_eq!(
-            status(
-                put_agent_inference(
-                    State(state),
-                    Path("iris".to_owned()),
-                    body(r#"{"model":"opus 4"}"#)
-                )
-                .await
-            ),
-            Some(StatusCode::BAD_REQUEST)
-        );
+        for bad in [r#"{"model":"opus 4"}"#, r#"{"model":"--print"}"#] {
+            assert_eq!(
+                status(
+                    put_agent_inference(State(state.clone()), Path("iris".to_owned()), body(bad))
+                        .await
+                ),
+                Some(StatusCode::BAD_REQUEST),
+                "{bad}"
+            );
+        }
     }
 
     #[tokio::test]