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Remove dead export isUsageExhausted() (superseded by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/streamed-output-evaluator.ts

@@ -3,13 +3,14 @@ import {
   isClaudeAuthExpiredMessage,
   isClaudeOrgAccessDeniedMessage,
   isClaudeUsageExhaustedMessage,
+  type AgentTriggerReason,
 } from './agent-error-detection.js';
 import type { AgentOutput } from './agent-runner.js';
 import { detectCodexRotationTrigger } from './codex-token-rotation.js';
 import { detectFallbackTrigger } from './provider-fallback.js';
 
 export interface StreamedTriggerReason {
-  reason: string;
+  reason: AgentTriggerReason;
   retryAfterMs?: number;
 }
 
@@ -51,7 +52,8 @@ export function evaluateStreamedOutput(
     !state.sawOutput &&
     typeof output.result === 'string'
   ) {
-    const triggerReason = isClaudeUsageExhaustedMessage(output.result)
+    const triggerReason: AgentTriggerReason | undefined =
+      isClaudeUsageExhaustedMessage(output.result)
       ? 'usage-exhausted'
       : isClaudeOrgAccessDeniedMessage(output.result)
         ? 'org-access-denied'