fix: planner validates bullet output, rejects meta-commentary

Sonnet sometimes narrates what it did instead of outputting the actual
STATE.md. Added validation that first line starts with '- ' and
strengthened the output-only instruction.

planner/planner-agent.sh

@@ -92,7 +92,7 @@ Update STATE.md by merging the new commits/issues into the existing snapshot.
 - No more than 30 bullet points — be concise and factual
 - If current STATE.md is empty, build the snapshot from scratch using the git log and issues
 
-Output ONLY the bullet list — no preamble, no markdown fences, no explanation."
+IMPORTANT: Output ONLY the updated bullet list. No summary of changes, no meta-commentary, no preamble, no markdown fences, no explanation of what you did. Just the bullets starting with '- '."
 
   PHASE1_OUTPUT=$(timeout "$CLAUDE_TIMEOUT" claude -p "$PHASE1_PROMPT" \
     --model sonnet \
@@ -106,6 +106,12 @@ Output ONLY the bullet list — no preamble, no markdown fences, no explanation.
     exit 1
   fi
 
+  # Validate output starts with bullet points (reject meta-commentary)
+  if ! echo "$PHASE1_OUTPUT" | head -1 | grep -q '^- '; then
+    log "ERROR: phase 1 output is not a bullet list — got: $(echo "$PHASE1_OUTPUT" | head -1)"
+    exit 1
+  fi
+
   # Atomic write
   TEMP_STATE=$(mktemp "${STATE_FILE}.XXXXXX")
   printf '%s\n' "$PHASE1_OUTPUT" > "$TEMP_STATE"