fix: use default port variable in prp-implement health check

Replace undefined PORT with ${PORT:-3000} fallback.

Author: Jamkris

commands/prp-implement.toml

@@ -173,7 +173,7 @@ SERVER_PID=$!
 # Wait for server to be ready (adjust port as needed)
 SERVER_READY=0
 for i in $(seq 1 30); do
-  if curl -sf http://localhost:PORT/health >/dev/null 2>&1; then
+  if curl -sf http://localhost:${PORT:-3000}/health >/dev/null 2>&1; then
     SERVER_READY=1
     break
   fi