u/cascade: add -json-output flag for structured synthesis output

Schema: {summary, answer, worker_results[], errors[]}

scripts/u/cascade

@@ -12,6 +12,8 @@
 #   -pm MODEL     planner model (default: $OLLIE_CASCADE_PLAN_MODEL or claude-sonnet-4-20250514)
 #   -wb BACKEND   worker backend (default: $OLLIE_CASCADE_BACKEND or ollama)
 #   -no-synthesize    skip the final planner synthesis step; just print worker results
+#   -json-output      request structured JSON from the synthesis step
+#                     schema: {summary, answer, worker_results[], errors[]}
 set -euo pipefail
 
 OLLIE="${OLLIE:-$HOME/mnt/ollie}"
@@ -25,6 +27,7 @@ WM="${OLLIE_CASCADE_MODEL:-qwen2.5-coder:14b}"
 die() { printf 'cascade: %s\n' "$*" >&2; exit 1; }
 
 synthesize=1
+json_output=0
 
 while [ $# -gt 0 ]; do
     case $1 in
@@ -33,6 +36,7 @@ while [ $# -gt 0 ]; do
         -wb) WB=$2; shift ;;
         -wm) WM=$2; shift ;;
         -no-synthesize) synthesize=0 ;;
+        -json-output) json_output=1 ;;
         --)  shift; break ;;
         -*)  die "unknown flag: $1" ;;
         *)   break ;;
@@ -111,13 +115,27 @@ if [ "$synthesize" -eq 0 ]; then
     printf '%s\n' "$results"
     [ "$failures" -gt 0 ] && exit 1
 else
-    "$B" -backend "$PB" -model "$PM" \
-        "Synthesize these worker results into a final, coherent answer to the original task. Note: workers marked [STATUS: failed] did not complete successfully — account for this in your synthesis.
+    synth_flags=(-backend "$PB" -model "$PM")
+    if [ "$json_output" -eq 1 ]; then
+        synth_flags+=(-json)
+        synth_prompt="Synthesize these worker results into a structured JSON response for the original task. Note: workers marked [STATUS: failed] did not complete successfully — include them in errors.
+
+Output schema: {\"summary\": \"<one-line summary>\", \"answer\": \"<full synthesized answer>\", \"worker_results\": [{\"id\": \"<worker-id>\", \"status\": \"done|failed\", \"output\": \"<worker output>\"}], \"errors\": [\"<description of any failures>\"]}
 
 ORIGINAL TASK:
 $task
 
 WORKER RESULTS:
 $results"
+    else
+        synth_prompt="Synthesize these worker results into a final, coherent answer to the original task. Note: workers marked [STATUS: failed] did not complete successfully — account for this in your synthesis.
+
+ORIGINAL TASK:
+$task
+
+WORKER RESULTS:
+$results"
+    fi
+    "$B" "${synth_flags[@]}" "$synth_prompt"
     [ "$failures" -gt 0 ] && exit 1
 fi