feat: DESIGN_SHOTGUN_LOOP uses file polling instead of stdout reading

Update the template resolver to instruct the agent to background $D serve
and poll for feedback-pending.json / feedback.json on a 5-second loop.
This matches the real-world pattern where Claude Code / Conductor agents
can't block on subprocess stdout.

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

scripts/resolvers/design.ts

@@ -854,12 +854,35 @@ $D compare --images "$_DESIGN_DIR/variant-A.png,$_DESIGN_DIR/variant-B.png,$_DES
 \`\`\`
 
 This command generates the board HTML, starts an HTTP server on a random port,
-and opens it in the user's default browser. It blocks until the user submits
-feedback. The feedback JSON is printed to stdout.
+and opens it in the user's default browser. **Run it in the background** with \`&\`
+because the agent needs to keep running while the user interacts with the board.
 
-**Reading the result:**
+**IMPORTANT: Reading feedback via file polling (not stdout):**
 
-The agent reads stdout. The JSON has this shape:
+The server writes feedback to files next to the board HTML. The agent polls for these:
+- \`$_DESIGN_DIR/feedback.json\` — written when user clicks Submit (final choice)
+- \`$_DESIGN_DIR/feedback-pending.json\` — written when user clicks Regenerate/Remix/More Like This
+
+**Polling loop** (run after launching \`$D serve\` in background):
+
+\`\`\`bash
+# Poll for feedback files every 5 seconds (up to 10 minutes)
+for i in $(seq 1 120); do
+  if [ -f "$_DESIGN_DIR/feedback.json" ]; then
+    echo "SUBMIT_RECEIVED"
+    cat "$_DESIGN_DIR/feedback.json"
+    break
+  elif [ -f "$_DESIGN_DIR/feedback-pending.json" ]; then
+    echo "REGENERATE_RECEIVED"
+    cat "$_DESIGN_DIR/feedback-pending.json"
+    rm "$_DESIGN_DIR/feedback-pending.json"
+    break
+  fi
+  sleep 5
+done
+\`\`\`
+
+The feedback JSON has this shape:
 \`\`\`json
 {
   "preferred": "A",
@@ -870,23 +893,23 @@ The agent reads stdout. The JSON has this shape:
 }
 \`\`\`
 
-**If \`"regenerated": true\`:**
+**If \`feedback-pending.json\` found (\`"regenerated": true\`):**
 1. Read \`regenerateAction\` from the JSON (\`"different"\`, \`"match"\`, \`"more_like_B"\`,
    \`"remix"\`, or custom text)
 2. If \`regenerateAction\` is \`"remix"\`, read \`remixSpec\` (e.g. \`{"layout":"A","colors":"B"}\`)
 3. Generate new variants with \`$D iterate\` or \`$D variants\` using updated brief
 4. Create new board: \`$D compare --images "..." --output "$_DESIGN_DIR/design-board.html"\`
-5. Reload the running server: parse the port from stderr (\`SERVE_STARTED: port=XXXXX\`),
-   then POST the new HTML:
-   \`curl -s -X POST http://localhost:PORT/api/reload -H 'Content-Type: application/json' -d '{"html":"$_DESIGN_DIR/design-board.html"}'\`
-6. The board auto-refreshes in the same browser tab. Wait for the next stdout line.
-7. Repeat until \`"regenerated": false\`.
+5. Parse the port from the \`$D serve\` stderr output (\`SERVE_STARTED: port=XXXXX\`),
+   then reload the board in the user's browser (same tab):
+   \`curl -s -X POST http://127.0.0.1:PORT/api/reload -H 'Content-Type: application/json' -d '{"html":"$_DESIGN_DIR/design-board.html"}'\`
+6. The board auto-refreshes. **Poll again** for the next feedback file.
+7. Repeat until \`feedback.json\` appears (user clicked Submit).
 
-**If \`"regenerated": false\`:**
+**If \`feedback.json\` found (\`"regenerated": false\`):**
 1. Read \`preferred\`, \`ratings\`, \`comments\`, \`overall\` from the JSON
 2. Proceed with the approved variant
 
-**If \`$D serve\` fails or times out:** Fall back to AskUserQuestion:
+**If \`$D serve\` fails or no feedback within 10 minutes:** Fall back to AskUserQuestion:
 "I've opened the design board. Which variant do you prefer? Any feedback?"
 
 **After receiving feedback (any path):** Output a clear summary confirming