fix(#734): agents/plugins show not-found envelopes gain message field; parity with skills show

rust/crates/commands/src/lib.rs

@@ -2459,6 +2459,8 @@ pub fn handle_agents_slash_command_json(args: Option<&str>, cwd: &Path) -> std::
                     "status": "error",
                     "error_kind": "agent_not_found",
                     "requested": name,
+                    // #734: parity with skills show which always emits a message field
+                    "message": format!("agent '{}' not found", name),
                 }));
             }
             Ok(render_agents_report_json_with_action(cwd, &matched, "show"))

rust/crates/rusty-claude-cli/src/main.rs

@@ -6140,6 +6140,8 @@ impl LiveCli {
                                 "status": "error",
                                 "error_kind": "plugin_not_found",
                                 "requested": name,
+                                // #734: parity with skills show which always emits a message field
+                                "message": format!("plugin '{}' not found", name),
                             });
                             println!("{}", serde_json::to_string_pretty(&obj)?);
                             return Ok(());