fix: document automation schedules as UTC

[debidong]

Summary

• Clarify that friendly automation schedule helpers and cron expressions are UTC
• Tell agents/users to convert local wall-clock requests before passing --at or --cron-expr
• Update the bundled Flashduty automation skill card and add help coverage

Test plan

• go test ./...
• go run ./internal/cmd/skilldoc check
• go build ./cmd/flashduty
• go run ./cmd/flashduty automation create --help
• go run ./cmd/flashduty automation update auto_123 --help