fix(design): drop gpt-image-2 tool model that 400s under the gpt-4o orchestrator

The Responses API rejects pairing a gpt-4o orchestrator with an
image_generation tool spec'd as model: "gpt-image-2" (400
invalid_request_error), which took every design image call offline —
generate, variants, iterate (both threaded and fresh paths), evolve,
and /design-shotgun (#1771). gpt-image-2 is only valid under a gpt-5
orchestrator; with gpt-4o the tool must omit the model field (defaults
to gpt-image-1).

Remove the model field at all five call sites and add a static-grep
tripwire test (design/test/image-gen-pairing.test.ts) that fails CI if
any design/src module reintroduces the gpt-4o + gpt-image-2 pairing.
Re-enabling gpt-image-2 later requires bumping the orchestrator off
gpt-4o in the same diff, which the tripwire permits.

Contributed by @Pablosinyores (PR #1773).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

design/src/evolve.ts

@@ -65,7 +65,7 @@ export async function evolve(options: EvolveOptions): Promise<void> {
       body: JSON.stringify({
         model: "gpt-4o",
         input: evolvedPrompt,
-        tools: [{ type: "image_generation", model: "gpt-image-2", size: "1536x1024", quality: "high" }],
+        tools: [{ type: "image_generation", size: "1536x1024", quality: "high" }],
       }),
       signal: controller.signal,
     });