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;