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/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