u/cascade: strict plan schema validation; clarify exit code contract

- Validate all entries have non-empty string id and instruction
- No synthesis: exit 1 on worker failures (partial results)
- Synthesis: exit 0 — planner accounts for failures in its answer

scripts/u/cascade

@@ -62,8 +62,8 @@ $task")
 
 # Validate we got JSON with correct schema.
 printf '%s' "$plan" | jq empty 2>/dev/null || die "planner did not return valid JSON: $plan"
-printf '%s' "$plan" | jq -e '[.[] | select(.id and .instruction)] | length > 0' >/dev/null \
-    || die "plan missing required fields (need .id and .instruction per entry)"
+printf '%s' "$plan" | jq -e 'type=="array" and length>0 and all(.[]; (.id|type=="string" and length>0) and (.instruction|type=="string" and length>0))' >/dev/null \
+    || die "plan schema invalid (need array of {id: string, instruction: string})"
 
 # Step 2: Spawn workers in parallel, collect session paths.
 count=$(printf '%s' "$plan" | jq 'length')
@@ -111,6 +111,9 @@ ${out}
 done
 
 # Step 5: Synthesize or just print results.
+# Exit code contract:
+#   No synthesis: exit 1 if any worker failed (caller must handle partial results).
+#   Synthesis: exit 0 — the planner accounts for failures in its answer.
 if [ "$synthesize" -eq 0 ]; then
     printf '%s\n' "$results"
     [ "$failures" -gt 0 ] && exit 1
@@ -137,5 +140,4 @@ WORKER RESULTS:
 $results"
     fi
     "$B" "${synth_flags[@]}" "$synth_prompt"
-    [ "$failures" -gt 0 ] && exit 1
 fi