fix: handoff to codex on persistent session failure

When Claude reviewer hits retryable session failure twice, hand off
to codex instead of silently failing. Adds 'session-failure' to
AgentTriggerReason and shouldHandoffToCodex check.

src/agent-error-detection.ts

@@ -101,7 +101,8 @@ export type AgentTriggerReason =
   | 'org-access-denied'
   | 'overloaded'
   | 'network-error'
-  | 'success-null-result';
+  | 'success-null-result'
+  | 'session-failure';
 
 export type ClaudeRotationReason = Extract<
   AgentTriggerReason,

src/message-agent-executor.ts

@@ -295,7 +295,8 @@ export async function runAgentForGroup(
       reason === '429' ||
       reason === 'usage-exhausted' ||
       reason === 'auth-expired' ||
-      reason === 'org-access-denied'
+      reason === 'org-access-denied' ||
+      reason === 'session-failure'
     );
   };
 
@@ -781,7 +782,7 @@ export async function runAgentForGroup(
         log.error(
           'Retryable Claude session failure persisted after fresh retry',
         );
-        return 'error';
+        return maybeHandoffAfterError('session-failure', primaryAttempt);
       }
     }