fix: write ALL feedback to disk so agent can poll in background mode

The agent backgrounds $D serve (Claude Code can't block on a subprocess
and do other work simultaneously). With stdout-only feedback delivery,
the agent never sees regenerate/remix feedback.

Fix: write feedback-pending.json (regenerate/remix) and feedback.json
(submit) to disk next to the board HTML. Agent polls the filesystem
instead of reading stdout. Both channels (stdout + disk) are always
active so foreground mode still works.

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

design/src/serve.ts

@@ -21,7 +21,14 @@
  *      │
  *      └──(timeout)──► exit 1
  *
- * Stdout: feedback JSON only (one line per feedback event)
+ * Feedback delivery (two channels, both always active):
+ *   Stdout: feedback JSON (one line per event) — for foreground mode
+ *   Disk:   feedback-pending.json (regenerate/remix) or feedback.json (submit)
+ *           written next to the HTML file — for background mode polling
+ *
+ * The agent typically backgrounds $D serve and polls for feedback-pending.json.
+ * When found: read it, delete it, generate new variants, POST /api/reload.
+ *
  * Stderr: structured telemetry (SERVE_STARTED, SERVE_FEEDBACK_RECEIVED, etc.)
  */
 
@@ -120,14 +127,17 @@ export async function serve(options: ServeOptions): Promise<void> {
 
     console.error(`SERVE_FEEDBACK_RECEIVED: type=${action}`);
 
-    // Print feedback JSON to stdout (agent reads this)
+    // Print feedback JSON to stdout (for foreground mode)
     console.log(JSON.stringify(body));
 
-    if (isSubmit) {
-      // Write feedback.json next to the HTML file
-      const feedbackPath = path.join(path.dirname(html), "feedback.json");
-      fs.writeFileSync(feedbackPath, JSON.stringify(body, null, 2));
+    // ALWAYS write feedback to disk so the agent can poll for it
+    // (agent typically backgrounds $D serve, can't read stdout)
+    const feedbackDir = path.dirname(html);
+    const feedbackFile = isSubmit ? "feedback.json" : "feedback-pending.json";
+    const feedbackPath = path.join(feedbackDir, feedbackFile);
+    fs.writeFileSync(feedbackPath, JSON.stringify(body, null, 2));
 
+    if (isSubmit) {
       state = "done";
       if (timeoutTimer) clearTimeout(timeoutTimer);
 

design/test/serve.test.ts

@@ -84,10 +84,10 @@ describe('Serve HTTP endpoints', () => {
             try { body = await req.json(); } catch { return Response.json({ error: 'Invalid JSON' }, { status: 400 }); }
             if (typeof body !== 'object' || body === null) return Response.json({ error: 'Expected JSON object' }, { status: 400 });
             const isSubmit = body.regenerated === false;
+            const feedbackFile = isSubmit ? 'feedback.json' : 'feedback-pending.json';
+            fs.writeFileSync(path.join(tmpDir, feedbackFile), JSON.stringify(body, null, 2));
             if (isSubmit) {
               state = 'done';
-              const feedbackPath = path.join(tmpDir, 'feedback.json');
-              fs.writeFileSync(feedbackPath, JSON.stringify(body, null, 2));
               return Response.json({ received: true, action: 'submitted' });
             }
             state = 'regenerating';
@@ -160,8 +160,12 @@ describe('Serve HTTP endpoints', () => {
     expect(written.ratings.A).toBe(4);
   });
 
-  test('POST /api/feedback with regenerate sets state to regenerating', async () => {
+  test('POST /api/feedback with regenerate sets state and writes feedback-pending.json', async () => {
     state = 'serving';
+    // Clean up any prior pending file
+    const pendingPath = path.join(tmpDir, 'feedback-pending.json');
+    if (fs.existsSync(pendingPath)) fs.unlinkSync(pendingPath);
+
     const feedback = {
       preferred: 'B',
       ratings: { A: 3, B: 5, C: 2 },
@@ -185,6 +189,12 @@ describe('Serve HTTP endpoints', () => {
     const progress = await fetch(`${baseUrl}/api/progress`);
     const pd = await progress.json();
     expect(pd.status).toBe('regenerating');
+
+    // Agent can poll for feedback-pending.json
+    expect(fs.existsSync(pendingPath)).toBe(true);
+    const pending = JSON.parse(fs.readFileSync(pendingPath, 'utf-8'));
+    expect(pending.regenerated).toBe(true);
+    expect(pending.regenerateAction).toBe('different');
   });
 
   test('POST /api/feedback with remix contains remixSpec', async () => {