Error out if no entrypoint or command is passed (drop claude default)

src/errors.rs

@@ -25,6 +25,7 @@ pub enum SandboxError {
     UnknownConfigKey(String),
     ConfigPathNotAbsolute(PathBuf),
     InvalidBwrapArg(String),
+    NoCommand,
 }
 
 impl std::fmt::Display for SandboxError {
@@ -69,6 +70,10 @@ impl std::fmt::Display for SandboxError {
             Self::InvalidBwrapArg(s) => {
                 write!(f, "invalid quoting in --bwrap-arg: {s}")
             }
+            Self::NoCommand => write!(
+                f,
+                "no command to run; specify a command via config, entrypoint, or pass one after --"
+            ),
         }
     }
 }