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Remove dead export isUsageExhausted() (superseded by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/provider-retry.ts

@@ -5,7 +5,11 @@
  * to eliminate the ~255-line structural duplication.
  */
 
-import { shouldRotateClaudeToken } from './agent-error-detection.js';
+import {
+  isNoFallbackCooldownReason,
+  shouldRotateClaudeToken,
+  type AgentTriggerReason,
+} from './agent-error-detection.js';
 import { logger } from './logger.js';
 import { getErrorMessage } from './utils.js';
 import {
@@ -21,7 +25,7 @@ import {
 // ── Types ────────────────────────────────────────────────────────
 
 export interface TriggerInfo {
-  reason: string;
+  reason: AgentTriggerReason;
   retryAfterMs?: number;
 }
 
@@ -163,11 +167,7 @@ export async function runClaudeRotationLoop(
   // ── All tokens exhausted ──
 
   // Usage/auth/org access failures: don't fall back to Kimi
-  if (
-    trigger.reason === 'usage-exhausted' ||
-    trigger.reason === 'auth-expired' ||
-    trigger.reason === 'org-access-denied'
-  ) {
+  if (isNoFallbackCooldownReason(trigger.reason)) {
     markPrimaryCooldown(trigger.reason, trigger.retryAfterMs);
     logger.info(
       { ...logContext, reason: trigger.reason },