feat: design binary variants + iterate commands

variants: generates N style variations with staggered parallel (1.5s
between launches, exponential backoff on 429). 7 built-in style
variations (bold, calm, warm, corporate, dark, playful + default).
Tested: 3/3 variants in 41.6s.

iterate: multi-turn design iteration using previous_response_id for
conversational threading. Falls back to re-generation with accumulated
feedback if threading doesn't retain visual context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

design/src/iterate.ts

@@ -0,0 +1,179 @@
+/**
+ * Multi-turn design iteration using OpenAI Responses API.
+ *
+ * Primary: uses previous_response_id for conversational threading.
+ * Fallback: if threading doesn't retain visual context, re-generates
+ * with original brief + accumulated feedback in a single prompt.
+ */
+
+import fs from "fs";
+import path from "path";
+import { requireApiKey } from "./auth";
+import { readSession, updateSession } from "./session";
+
+export interface IterateOptions {
+  session: string;   // Path to session JSON file
+  feedback: string;  // User feedback text
+  output: string;    // Output path for new PNG
+}
+
+/**
+ * Iterate on an existing design using session state.
+ */
+export async function iterate(options: IterateOptions): Promise<void> {
+  const apiKey = requireApiKey();
+  const session = readSession(options.session);
+
+  console.error(`Iterating on session ${session.id}...`);
+  console.error(`  Previous iterations: ${session.feedbackHistory.length}`);
+  console.error(`  Feedback: "${options.feedback}"`);
+
+  const startTime = Date.now();
+
+  // Try multi-turn with previous_response_id first
+  let success = false;
+  let responseId = "";
+
+  try {
+    const result = await callWithThreading(apiKey, session.lastResponseId, options.feedback);
+    responseId = result.responseId;
+
+    fs.mkdirSync(path.dirname(options.output), { recursive: true });
+    fs.writeFileSync(options.output, Buffer.from(result.imageData, "base64"));
+    success = true;
+  } catch (err: any) {
+    console.error(`  Threading failed: ${err.message}`);
+    console.error("  Falling back to re-generation with accumulated feedback...");
+
+    // Fallback: re-generate with original brief + all feedback
+    const accumulatedPrompt = buildAccumulatedPrompt(
+      session.originalBrief,
+      [...session.feedbackHistory, options.feedback]
+    );
+
+    const result = await callFresh(apiKey, accumulatedPrompt);
+    responseId = result.responseId;
+
+    fs.mkdirSync(path.dirname(options.output), { recursive: true });
+    fs.writeFileSync(options.output, Buffer.from(result.imageData, "base64"));
+    success = true;
+  }
+
+  if (success) {
+    const elapsed = ((Date.now() - startTime) / 1000).toFixed(1);
+    const size = fs.statSync(options.output).size;
+    console.error(`Generated (${elapsed}s, ${(size / 1024).toFixed(0)}KB) → ${options.output}`);
+
+    // Update session
+    updateSession(session, responseId, options.feedback, options.output);
+
+    console.log(JSON.stringify({
+      outputPath: options.output,
+      sessionFile: options.session,
+      responseId,
+      iteration: session.feedbackHistory.length + 1,
+    }, null, 2));
+  }
+}
+
+async function callWithThreading(
+  apiKey: string,
+  previousResponseId: string,
+  feedback: string,
+): Promise<{ responseId: string; imageData: string }> {
+  const controller = new AbortController();
+  const timeout = setTimeout(() => controller.abort(), 120_000);
+
+  try {
+    const response = await fetch("https://api.openai.com/v1/responses", {
+      method: "POST",
+      headers: {
+        "Authorization": `Bearer ${apiKey}`,
+        "Content-Type": "application/json",
+      },
+      body: JSON.stringify({
+        model: "gpt-4o",
+        input: `Based on the previous design, make these changes: ${feedback}`,
+        previous_response_id: previousResponseId,
+        tools: [{ type: "image_generation", size: "1536x1024", quality: "high" }],
+      }),
+      signal: controller.signal,
+    });
+
+    if (!response.ok) {
+      const error = await response.text();
+      throw new Error(`API error (${response.status}): ${error.slice(0, 300)}`);
+    }
+
+    const data = await response.json() as any;
+    const imageItem = data.output?.find((item: any) => item.type === "image_generation_call");
+
+    if (!imageItem?.result) {
+      throw new Error("No image data in threaded response");
+    }
+
+    return { responseId: data.id, imageData: imageItem.result };
+  } finally {
+    clearTimeout(timeout);
+  }
+}
+
+async function callFresh(
+  apiKey: string,
+  prompt: string,
+): Promise<{ responseId: string; imageData: string }> {
+  const controller = new AbortController();
+  const timeout = setTimeout(() => controller.abort(), 120_000);
+
+  try {
+    const response = await fetch("https://api.openai.com/v1/responses", {
+      method: "POST",
+      headers: {
+        "Authorization": `Bearer ${apiKey}`,
+        "Content-Type": "application/json",
+      },
+      body: JSON.stringify({
+        model: "gpt-4o",
+        input: prompt,
+        tools: [{ type: "image_generation", size: "1536x1024", quality: "high" }],
+      }),
+      signal: controller.signal,
+    });
+
+    if (!response.ok) {
+      const error = await response.text();
+      throw new Error(`API error (${response.status}): ${error.slice(0, 300)}`);
+    }
+
+    const data = await response.json() as any;
+    const imageItem = data.output?.find((item: any) => item.type === "image_generation_call");
+
+    if (!imageItem?.result) {
+      throw new Error("No image data in fresh response");
+    }
+
+    return { responseId: data.id, imageData: imageItem.result };
+  } finally {
+    clearTimeout(timeout);
+  }
+}
+
+function buildAccumulatedPrompt(originalBrief: string, feedback: string[]): string {
+  const lines = [
+    originalBrief,
+    "",
+    "Previous feedback (apply all of these changes):",
+  ];
+
+  feedback.forEach((f, i) => {
+    lines.push(`${i + 1}. ${f}`);
+  });
+
+  lines.push(
+    "",
+    "Generate a new mockup incorporating ALL the feedback above.",
+    "The result should look like a real production UI, not a wireframe."
+  );
+
+  return lines.join("\n");
+}