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,