Add Temporal worker and test client programs

Creates:
- cmd/worker/main.go: Worker that registers workflows and activities
- cmd/test-workflow/main.go: Test client to trigger workflows

Adds go.temporal.io/sdk dependency to go.mod.

internal/workflow/helloworld.go

@@ -0,0 +1,56 @@
+package workflow
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"go.temporal.io/sdk/workflow"
+)
+
+// HelloWorldWorkflow is a simple hello world workflow
+func HelloWorldWorkflow(ctx workflow.Context, name string) (string, error) {
+	opts := workflow.ActivityOptions{
+		StartToCloseTimeout: time.Minute,
+	}
+	ctx = workflow.WithActivityOptions(ctx, opts)
+
+	var result string
+	if err := workflow.ExecuteActivity(ctx, GreetActivity, name).Get(ctx, &result); err != nil {
+		return "", err
+	}
+	return result, nil
+}
+
+// GreetActivity greets someone
+func GreetActivity(ctx context.Context, name string) (string, error) {
+	return fmt.Sprintf("Hello, %s!", name), nil
+}
+
+// ValidateOrderActivity validates an order
+func ValidateOrderActivity(ctx context.Context, orderID string) (bool, error) {
+	// Simulate validation
+	if orderID == "" {
+		return false, nil
+	}
+	return true, nil
+}
+
+// ProcessPaymentActivity processes payment
+func ProcessPaymentActivity(ctx context.Context, orderID string) (string, error) {
+	// Simulate payment processing
+	return fmt.Sprintf("payment-%s", orderID[:min(len(orderID), 3)]), nil
+}
+
+// NotifyCustomerActivity sends notification
+func NotifyCustomerActivity(ctx context.Context, orderID string) (string, error) {
+	// Simulate notification
+	return fmt.Sprintf("notified for order %s", orderID), nil
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}