Expose workflow.Cause to retrieve cancellation cause

[Copilot]

Cause(ctx) only lived in the internal sync package, so workflows had no way to retrieve the error passed to WithCancelCause. ctx.Err() only yields the generic context canceled, losing the underlying cause.

Changes

• workflow/context.go: Add exported Cause(c Context) error wrapping sync.Cause, following the existing WithCancel/WithCancelCause delegation pattern.
• workflow/context_test.go: Add TestCause covering the pre-cancellation (nil) and post-cancellation cases.

Usage

func Workflow(ctx workflow.Context, input string) (string, error) {
    ctx, cancel := workflow.WithCancelCause(ctx)
    cancel(errors.New("This workflow is canceled"))
    return "", workflow.Cause(ctx) // returns the original error instead of "context canceled"
}