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/commands.ts

@@ -0,0 +1,62 @@
+/**
+ * Command registry — single source of truth for all design commands.
+ *
+ * Dependency graph:
+ *   commands.ts ──▶ cli.ts (runtime dispatch)
+ *              ──▶ gen-skill-docs.ts (doc generation)
+ *              ──▶ tests (validation)
+ *
+ * Zero side effects. Safe to import from build scripts and tests.
+ */
+
+export const COMMANDS = new Map<string, {
+  description: string;
+  usage: string;
+  flags?: string[];
+}>([
+  ["generate", {
+    description: "Generate a UI mockup from a design brief",
+    usage: "generate --brief \"...\" --output /path.png",
+    flags: ["--brief", "--brief-file", "--output", "--check", "--retry", "--size", "--quality"],
+  }],
+  ["variants", {
+    description: "Generate N design variants from a brief",
+    usage: "variants --brief \"...\" --count 3 --output-dir /path/",
+    flags: ["--brief", "--brief-file", "--count", "--output-dir", "--size", "--quality", "--viewports"],
+  }],
+  ["iterate", {
+    description: "Iterate on an existing mockup with feedback",
+    usage: "iterate --session /path/session.json --feedback \"...\" --output /path.png",
+    flags: ["--session", "--feedback", "--output"],
+  }],
+  ["check", {
+    description: "Vision-based quality check on a mockup",
+    usage: "check --image /path.png --brief \"...\"",
+    flags: ["--image", "--brief"],
+  }],
+  ["compare", {
+    description: "Generate HTML comparison board for user review",
+    usage: "compare --images /path/*.png --output /path/board.html",
+    flags: ["--images", "--output"],
+  }],
+  ["diff", {
+    description: "Visual diff between two mockups",
+    usage: "diff --before old.png --after new.png",
+    flags: ["--before", "--after", "--output"],
+  }],
+  ["evolve", {
+    description: "Generate improved mockup from existing screenshot",
+    usage: "evolve --screenshot current.png --brief \"make it calmer\" --output /path.png",
+    flags: ["--screenshot", "--brief", "--output"],
+  }],
+  ["verify", {
+    description: "Compare live site screenshot against approved mockup",
+    usage: "verify --mockup approved.png --screenshot live.png",
+    flags: ["--mockup", "--screenshot", "--output"],
+  }],
+  ["setup", {
+    description: "Guided API key setup + smoke test",
+    usage: "setup",
+    flags: [],
+  }],
+]);