Go error handling patterns, wrapping, sentinel errors, custom error types, and the errors package. Grounded in Effective Go, Go Code Review Comments, and production-proven idioms. Use when implementing error handling, designing error types, debugging error chains, or reviewing error handling patterns. Trigger examples: "handle errors", "error wrapping", "custom error type", "sentinel errors", "errors.Is", "errors.As". Not for: panic/recover in middleware (go-api-design), test assertions (go-test-quality).
