topdata: skip .md doc files in asset validation and packaging

Docs like AGENTS.md/CLAUDE.md under assets/ broke sow-topdata builds
with "unsupported topdata asset HAK resource extension". Skip .md in
both the validator and HAK package walker.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/topdata/topdata_test.go

@@ -15115,3 +15115,23 @@ func writeBytes(t *testing.T, path string, content []byte) {
 		t.Fatalf("write %s: %v", path, err)
 	}
 }
+
+func TestValidateTopPackageAssetsSkipsMarkdown(t *testing.T) {
+	dir := t.TempDir()
+	assets := filepath.Join(dir, "assets", "gui")
+	if err := os.MkdirAll(assets, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(assets, "AGENTS.md"), []byte("docs"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	dataDir := filepath.Join(dir, "data")
+	if err := os.MkdirAll(dataDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	var report ValidationReport
+	validateTopPackageAssets(dir, dataDir, &report)
+	for _, d := range report.Diagnostics {
+		t.Errorf("unexpected diagnostic: %s: %s", d.Path, d.Message)
+	}
+}