style(orchestrator): post-S4 clippy nits — manual_strip + flatten test modules

Three clippy warnings surfaced by the final-review workspace clippy
pass that the per-task gates didn't catch:

- design_system.rs::extract_code_fence — replace manual slice
  `&after_open[4..]` with `strip_prefix("json")`. Same behaviour,
  cleaner intent, satisfies clippy::manual_strip.
- design_system_tests.rs + visual_ref_tests.rs — flatten the inner
  `#[cfg(test)] mod tests { ... }` wrapper. The files are already
  wired via `#[path = "..."] mod tests;` in their parent modules
  (design_system.rs / visual_ref.rs), so the inner wrapper was double
  nesting (clippy::module_inception). Matches the established
  pattern in concurrent_tests.rs / plan_repair_tests.rs / run_tests.rs.

574 tests still pass. clippy + fmt + cargo build --workspace clean.

crates/op-orchestrator/src/design_system.rs

@@ -250,11 +250,7 @@ fn extract_code_fence(text: &str) -> Option<&str> {
     let after_open = &text[fence_start + 3..];
 
     // Skip optional "json" language tag and the following newline
-    let content_start = if after_open.starts_with("json") {
-        &after_open[4..]
-    } else {
-        after_open
-    };
+    let content_start = after_open.strip_prefix("json").unwrap_or(after_open);
 
     // Skip leading newline
     let content_start = content_start.strip_prefix('\n').unwrap_or(content_start);