brokerd: escape container errors in the log; prefix and quote two messages

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/src/config.rs

@@ -176,7 +176,7 @@ impl Config {
         }
         if let Some(runner) = &config.runner {
             let where_image = format!(
-                "[runner] image is {}; it must be named by digest: <name>@sha256:<64 hex digits>",
+                "[runner] image is {:?}; it must be named by digest: <name>@sha256:<64 hex digits>",
                 runner.image
             );
             let (name, hex) = match runner.image.rsplit_once("@sha256:") {
@@ -198,7 +198,7 @@ impl Config {
                 return Err(ConfigError::Invalid(
                     path.to_path_buf(),
                     format!(
-                        "[runner] memory is {}; it must be a number and one of b, k, m, g",
+                        "[runner] memory is {:?}; it must be a number and one of b, k, m, g",
                         runner.memory
                     ),
                 ));