Skip worker launch when build_*_prompt returns empty content

scripts/opencode-builder.sh

@@ -1185,7 +1185,7 @@ main_loop() {
               local tmp_prompt
               tmp_prompt=$(mktemp "/tmp/ocb-${worker_tag}-${$}.XXXXXX.txt") || continue
 
-              local prompt_text
+              local prompt_text=""
               case "$agent" in
                 *implementation*) prompt_text=$(build_impl_prompt "$item" "$kind" "$work_num") ;;
                 *merge*)          prompt_text=$(build_merge_prompt "$item" "$work_num") ;;
@@ -1193,6 +1193,15 @@ main_loop() {
                 *grooming*)       prompt_text=$(build_groom_prompt "$item" "$kind" "$work_num") ;;
               esac
 
+              # Skip if the build function returned an empty prompt — sending
+              # an empty body to prompt_async results in "no user message
+              # found in stream" errors on the server.
+              if [[ -z "$prompt_text" ]]; then
+                log_warn "[pool:${prefix}]   built empty prompt for ${kind} ${work_num} (${agent}) — skipping launch"
+                rm -f "$tmp_prompt"
+                continue
+              fi
+
               printf '%s' "$prompt_text" > "$tmp_prompt"
 
               $SESSION_START --server "${BASE}" --tag "$worker_tag" --agent "$agent" --prompt-file "$tmp_prompt" --restart \