feat: design memory — extract visual language from mockups into DESIGN.md

New `$D extract` command: sends approved mockup to GPT-4o vision,
extracts color palette, typography, spacing, and layout patterns,
writes/updates DESIGN.md with an "Extracted Design Language" section.

Progressive constraint: if DESIGN.md exists, future mockup briefs
include it as style context. If no DESIGN.md, explorations run wide.
readDesignConstraints() reads existing DESIGN.md for brief construction.

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

design/src/cli.ts

@@ -19,6 +19,7 @@ import { compare } from "./compare";
 import { variants } from "./variants";
 import { iterate } from "./iterate";
 import { resolveApiKey, saveApiKey } from "./auth";
+import { extractDesignLanguage, updateDesignMd } from "./memory";
 
 function parseArgs(argv: string[]): { command: string; flags: Record<string, string | boolean> } {
   const args = argv.slice(2); // skip bun/node and script path
@@ -160,6 +161,23 @@ async function main(): Promise<void> {
       });
       break;
 
+    case "extract": {
+      const imagePath = flags.image as string;
+      if (!imagePath) {
+        console.error("--image is required");
+        process.exit(1);
+      }
+      console.error(`Extracting design language from ${imagePath}...`);
+      const extracted = await extractDesignLanguage(imagePath);
+      const proc = Bun.spawn(["git", "rev-parse", "--show-toplevel"]);
+      const repoRoot = (await new Response(proc.stdout).text()).trim();
+      if (repoRoot) {
+        updateDesignMd(repoRoot, extracted, imagePath);
+      }
+      console.log(JSON.stringify(extracted, null, 2));
+      break;
+    }
+
     case "diff":
     case "evolve":
     case "verify":