ci: harden cto-review.yml severity KeyError (v0.18.3)

QA (seq 43) identified a content-triggered KeyError in the CTO-Review
postprocessor: when a model returns a concerns[] item without a 'severity'
key, line 454 crashed with KeyError instead of degrading.

  sev = c[severity].upper()        # before — hard KeyError
  sev = c.get("severity", "UNKNOWN").upper()  # after — degrades gracefully

The JSON schema marks severity as required but the postprocessor does not
enforce it — a model deviating on a large diff (91K bytes for f10e177 vs
41K for 02e9aab) triggered the crash. The fix surfaces UNKNOWN-severity
concerns rather than killing the check.

No other workflow changes.

Agent-Session: prab-v0183-20260612

.github/workflows/cto-review.yml

@@ -451,7 +451,7 @@ jobs:
               if rev.get("concerns"):
                   lines.append("**Concerns:**")
                   for c in rev["concerns"]:
-                      sev = c["severity"].upper()
+                      sev = c.get("severity", "UNKNOWN").upper()
                       f = f"`{c['file']}`" if c.get("file") else ""
                       lines.append(f"- **{sev}** {f} — {c['issue']}")
                   lines.append("")