fix: remove unused field and add missing Context import

src/digest_engine/program.rs

@@ -26,7 +26,6 @@ impl DigestEngineProgram {
             program: program_path.unwrap_or(program.to_string()),
             args: args.iter().map(|s| s.to_string()).collect(),
             supported,
-            process: None,
         }
     }
 }