chore(infra): point migration helper preflight at 'codex migrate'

Now that 'codex migrate' applies the idempotent schema via a privileged role,
recommend it (with MIGRATION_DATABASE_URL) as the primary schema-sync fix in the
preflight abort message, keeping the manual owner-ALTER as a fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

infra/reingest_canonical_ids.sh

@@ -76,11 +76,13 @@ PYEOF
 )"
 if [[ "$SECTION_OK" != "yes" ]]; then
   echo "ABORT — schema not ready: chunks.section is missing (audit M-1)."
-  echo "The app user cannot add it (chunks is owned by 'postgres'). Apply it as"
-  echo "the table owner, then re-run this script:"
+  echo "The app user cannot add it (core tables are owned by 'postgres'). Sync the"
+  echo "schema with a privileged role, then re-run this script. Either:"
   echo ""
-  echo "    ssh alfred@192.168.178.103 \\"
-  echo "      \"sudo -u postgres psql -d papers -c \\\\\"ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT;\\\\\"\""
+  echo "  (a) MIGRATION_DATABASE_URL=postgresql://postgres:PASS@HOST:PORT/papers \\"
+  echo "        \"$CODEX_DIR/.venv/bin/codex\" migrate     # applies schema.sql idempotently"
+  echo "  (b) sudo -u postgres psql -d papers \\"
+  echo "        -c \"ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT;\""
   echo ""
   echo "Nothing was changed — no TRUNCATE was issued."
   exit 1