refactor: extract shared error detection and token rotation utilities (SSOT)

- Create agent-error-detection.ts: single source for error classification
  (isClaudeUsageExhaustedMessage, isClaudeAuthExpiredMessage, classifyAgentError, etc.)
- Create token-rotation-base.ts: shared rotation algorithm
  (parseRetryAfterFromError, computeCooldownUntil, findNextAvailable)
- Remove 6 duplicated functions from message-agent-executor and task-scheduler
- Delegate detectFallbackTrigger/detectCodexRotationTrigger to shared classifiers
- Split canFallback into isClaudeAgent/canRotateToken/canFallback (3-way separation)
- Cache env.ts with getEnv() to eliminate repeated disk I/O
- Migrate config, discord, provider-fallback, token-rotation, memento-client to getEnv()

Net: -146 lines, 0 new dependencies, 354/354 tests passing

src/codex-token-rotation.ts

@@ -204,7 +204,6 @@ function loadCodexState(): void {
   }
 }
 
-
 /** Get the auth.json path for the current active account. */
 export function getActiveCodexAuthPath(): string | null {
   if (accounts.length === 0) return null;

src/config.ts

@@ -70,8 +70,7 @@ export const STATUS_UPDATE_INTERVAL = 10000; // 10s
 export const USAGE_UPDATE_INTERVAL = 300000; // 5 minutes
 export const STATUS_SHOW_ROOMS =
   (getEnv('STATUS_SHOW_ROOMS') || 'true') !== 'false';
-export const USAGE_DASHBOARD_ENABLED =
+export const USAGE_DASHBOARD_ENABLED = getEnv('USAGE_DASHBOARD') === 'true';
-  getEnv('USAGE_DASHBOARD') === 'true';
 
 // Timezone for scheduled tasks (cron expressions, etc.)
 // Uses system timezone by default

src/provider-fallback.ts

@@ -82,10 +82,7 @@ function loadConfig(): FallbackConfig {
     authToken,
     model,
     smallModel: getEnv('FALLBACK_SMALL_MODEL') || model,
-    defaultCooldownMs: parseInt(
+    defaultCooldownMs: parseInt(getEnv('FALLBACK_COOLDOWN_MS') || '600000', 10),
-      getEnv('FALLBACK_COOLDOWN_MS') || '600000',
-      10,
-    ),
   };
 
   if (_config.enabled) {

src/task-scheduler.ts

@@ -78,7 +78,6 @@ export {
   shouldUseTaskScopedSession,
 } from './task-watch-status.js';
 
-
 /**
  * Compute the next run time for a recurring task, anchored to the
  * task's scheduled time rather than Date.now() to prevent cumulative

src/token-rotation.ts

@@ -105,7 +105,6 @@ function loadState(): void {
   }
 }
 
-
 /** Get the current active token. */
 export function getCurrentToken(): string | undefined {
   if (tokens.length === 0) return process.env.CLAUDE_CODE_OAUTH_TOKEN;