feat: design binary core — generate, check, compare commands

Stateless CLI (design/dist/design) wrapping OpenAI Responses API for
UI mockup generation. Three working commands:

- generate: brief -> PNG mockup via gpt-4o + image_generation tool
- check: vision-based quality gate via GPT-4o (text readability, layout
  completeness, visual coherence)
- compare: generates self-contained HTML comparison board with star
  ratings, radio Pick, per-variant feedback, regenerate controls,
  and Submit button that writes structured JSON for agent polling

Auth reads from ~/.gstack/openai.json (0600), falls back to
OPENAI_API_KEY env var. Compiled separately from browse binary
(openai added to devDependencies, not runtime deps).

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

design/src/session.ts

@@ -0,0 +1,79 @@
+/**
+ * Session state management for multi-turn design iteration.
+ * Session files are JSON in /tmp, keyed by PID + timestamp.
+ */
+
+import fs from "fs";
+import path from "path";
+
+export interface DesignSession {
+  id: string;
+  lastResponseId: string;
+  originalBrief: string;
+  feedbackHistory: string[];
+  outputPaths: string[];
+  createdAt: string;
+  updatedAt: string;
+}
+
+/**
+ * Generate a unique session ID from PID + timestamp.
+ */
+export function createSessionId(): string {
+  return `${process.pid}-${Date.now()}`;
+}
+
+/**
+ * Get the file path for a session.
+ */
+export function sessionPath(sessionId: string): string {
+  return path.join("/tmp", `design-session-${sessionId}.json`);
+}
+
+/**
+ * Create a new session after initial generation.
+ */
+export function createSession(
+  responseId: string,
+  brief: string,
+  outputPath: string,
+): DesignSession {
+  const id = createSessionId();
+  const session: DesignSession = {
+    id,
+    lastResponseId: responseId,
+    originalBrief: brief,
+    feedbackHistory: [],
+    outputPaths: [outputPath],
+    createdAt: new Date().toISOString(),
+    updatedAt: new Date().toISOString(),
+  };
+
+  fs.writeFileSync(sessionPath(id), JSON.stringify(session, null, 2));
+  return session;
+}
+
+/**
+ * Read an existing session from disk.
+ */
+export function readSession(sessionFilePath: string): DesignSession {
+  const content = fs.readFileSync(sessionFilePath, "utf-8");
+  return JSON.parse(content);
+}
+
+/**
+ * Update a session with new iteration data.
+ */
+export function updateSession(
+  session: DesignSession,
+  responseId: string,
+  feedback: string,
+  outputPath: string,
+): void {
+  session.lastResponseId = responseId;
+  session.feedbackHistory.push(feedback);
+  session.outputPaths.push(outputPath);
+  session.updatedAt = new Date().toISOString();
+
+  fs.writeFileSync(sessionPath(session.id), JSON.stringify(session, null, 2));
+}