Implement artifact-level render command with Markdown output and update docs

internal/render/registry_test.go

@@ -0,0 +1,22 @@
+package render
+
+import "testing"
+
+func TestRegistryResolvesMarkdownRenderer(t *testing.T) {
+	registry := NewRegistry()
+	renderer, err := registry.Resolve(FormatMarkdown)
+	if err != nil {
+		t.Fatalf("resolve markdown renderer: %v", err)
+	}
+	if renderer == nil {
+		t.Fatal("expected renderer")
+	}
+}
+
+func TestRegistryRejectsUnknownRenderer(t *testing.T) {
+	registry := NewRegistry()
+	_, err := registry.Resolve("txt")
+	if err == nil {
+		t.Fatal("expected unsupported format error")
+	}
+}