refactor: introduce AgentTriggerReason type union as SSOT for error reason strings

Replace scattered reason string literals with a centralized type hierarchy:
- AgentTriggerReason: all possible trigger reasons
- ClaudeRotationReason, CodexRotationReason, NoFallbackCooldownReason:
  derived subtypes for specific contexts
- AgentErrorClassification, FallbackTriggerResult, CodexRotationTriggerResult:
  discriminated unions for compile-time narrowing

Remove dead export isUsageExhausted() (superseded by
isPrimaryNoFallbackCooldownActive). Replace NO_FALLBACK_COOLDOWN_REASONS
Set with isNoFallbackCooldownReason() type guard.

Add tests for agent-error-detection and provider-retry (423 total passing).

Known limitation: 6 `as CodexRotationReason` casts in Codex rotation paths
where streamed trigger reason is AgentTriggerReason but runtime value is
always CodexRotationReason-compatible.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/codex-token-rotation.ts

@@ -17,6 +17,7 @@ import path from 'path';
 import {
   classifyAgentError,
   classifyCodexAuthError,
+  type CodexRotationReason,
 } from './agent-error-detection.js';
 import { DATA_DIR } from './config.js';
 import { logger } from './logger.js';
@@ -42,10 +43,15 @@ interface CodexAccount {
   resetD7At?: string;
 }
 
-export interface CodexRotationTriggerResult {
-  shouldRotate: boolean;
-  reason: string;
-}
+export type CodexRotationTriggerResult =
+  | {
+      shouldRotate: false;
+      reason: '';
+    }
+  | {
+      shouldRotate: true;
+      reason: CodexRotationReason;
+    };
 
 function parseJwtAuth(idToken: string): {
   planType: string;