feat: add $D serve command for HTTP-based comparison board feedback

The comparison board feedback loop was fundamentally broken: browse blocks
file:// URLs (url-validation.ts:71), so $B goto file://board.html always
fails. The fallback open + $B eval polls a different browser instance.

$D serve fixes this by serving the board over HTTP on localhost. The server
is stateful: stays alive across regeneration rounds, exposes /api/progress
for the board to poll, and accepts /api/reload from the agent to swap in
new board HTML. Stdout carries feedback JSON only; stderr carries telemetry.

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

design/src/cli.ts

@@ -23,6 +23,7 @@ import { extractDesignLanguage, updateDesignMd } from "./memory";
 import { diffMockups, verifyAgainstMockup } from "./diff";
 import { evolve } from "./evolve";
 import { generateDesignToCodePrompt } from "./design-to-code";
+import { serve } from "./serve";
 
 function parseArgs(argv: string[]): { command: string; flags: Record<string, string | boolean> } {
   const args = argv.slice(2); // skip bun/node and script path
@@ -134,10 +135,15 @@ async function main(): Promise<void> {
       // Parse --images as glob or multiple files
       const imagesArg = flags.images as string;
       const images = await resolveImagePaths(imagesArg);
-      compare({
-        images,
-        output: (flags.output as string) || "/tmp/gstack-design-board.html",
-      });
+      const outputPath = (flags.output as string) || "/tmp/gstack-design-board.html";
+      compare({ images, output: outputPath });
+      // If --serve flag is set, start HTTP server for the board
+      if (flags.serve) {
+        await serve({
+          html: outputPath,
+          timeout: flags.timeout ? parseInt(flags.timeout as string) : 600,
+        });
+      }
       break;
     }
 
@@ -230,6 +236,13 @@ async function main(): Promise<void> {
         output: (flags.output as string) || "/tmp/gstack-evolved.png",
       });
       break;
+
+    case "serve":
+      await serve({
+        html: flags.html as string,
+        timeout: flags.timeout ? parseInt(flags.timeout as string) : 600,
+      });
+      break;
   }
 }