- Add WorkErrorCode type with RATE_LIMITED, AUTH_FAILED, TIMEOUT, STALE_WORKER, AGENT_ERROR, INVALID_SPEC
- Add ClassifyAgentError function to detect error patterns from stderr
- Add error_code column to work_queue table (migration 016)
- Add FailWithCode method to WorkQueue interface and implementations
- Update RequeueStaleWithIDs to mark permanently failed tasks with STALE_WORKER
- Add ErrorCode to BuildResult for API responses
- Update work executor to classify errors before failing tasks
This enables users to see actual failure reasons (e.g., "RATE_LIMITED") instead of
builds stuck in "running" state forever when Claude hits rate limits.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
