(workflow) add simple harness workflow for manual testing

prompts/registry.go

@@ -1,3 +1,28 @@
 package prompts
 
-// Empty stub - will be filled in T0.5
+import (
+	"embed"
+	"fmt"
+	"strings"
+	"text/template"
+)
+
+//go:embed planner/default.tmpl judge/default.tmpl implementer/default.tmpl
+var templates embed.FS
+
+// Render renders a template with the given variables.
+func Render(templateRef string, variables map[string]any) (string, error) {
+	// Load template from embedded files
+	tmpl, err := template.ParseFS(templates, templateRef)
+	if err != nil {
+		return "", fmt.Errorf("failed to parse template %s: %w", templateRef, err)
+	}
+
+	// Render the template
+	var result strings.Builder
+	if err := tmpl.Execute(&result, variables); err != nil {
+		return "", fmt.Errorf("failed to render template %s: %w", templateRef, err)
+	}
+
+	return result.String(), nil
+}