feat: add show-and-wait.sh helper, fix race condition

- New show-and-wait.sh combines write + wait into one command
- Uses polling instead of tail -f (which hangs on macOS)
- Docs updated: start watcher BEFORE writing screen to avoid race
- Reduces terminal noise by consolidating operations

lib/brainstorm-server/show-and-wait.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+# Write HTML to screen and wait for user feedback
+# Usage: show-and-wait.sh <screen_dir> < html_content
+#
+# Reads HTML from stdin, writes to screen_file, waits for feedback.
+# Outputs the feedback JSON when user sends from browser.
+
+SCREEN_DIR="${1:?Usage: show-and-wait.sh <screen_dir>}"
+SCREEN_FILE="${SCREEN_DIR}/screen.html"
+LOG_FILE="${SCREEN_DIR}/.server.log"
+
+if [[ ! -d "$SCREEN_DIR" ]]; then
+  echo '{"error": "Screen directory not found"}' >&2
+  exit 1
+fi
+
+# Write HTML from stdin to screen file
+cat > "$SCREEN_FILE"
+
+# Record current position in log file
+LOG_POS=$(wc -l < "$LOG_FILE" 2>/dev/null || echo 0)
+
+# Poll for new lines containing the event
+while true; do
+  # Check for new matching lines since our starting position
+  RESULT=$(tail -n +$((LOG_POS + 1)) "$LOG_FILE" 2>/dev/null | grep -m 1 "send-to-claude")
+  if [[ -n "$RESULT" ]]; then
+    echo "$RESULT"
+    exit 0
+  fi
+  sleep 0.2
+done