Add full test suite with unit, integration, and E2E smoke tests.

Introduce test.bat orchestrating Go/Vitest/Playwright phases, expand coverage across server, agent, and dashboard, and document in tests/README.md.

server/internal/builder/path_test.go

@@ -0,0 +1,52 @@
+package builder
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSafePathUnderRootAllowsNormalFile(t *testing.T) {
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, "readme.txt"), []byte("ok"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := safePathUnderRoot(root, "readme.txt")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if _, err := os.Stat(got); err != nil {
+		t.Fatalf("resolved path missing: %v", err)
+	}
+}
+
+func TestSafePathUnderRootRejectsTraversal(t *testing.T) {
+	root := t.TempDir()
+	cases := []string{"../secret.txt", "..", "..\\windows\\system32", "foo/../../etc/passwd"}
+	for _, name := range cases {
+		if _, err := safePathUnderRoot(root, name); err == nil {
+			t.Fatalf("expected rejection for %q", name)
+		}
+	}
+}
+
+func TestSafePathUnderRootRejectsEscapeViaJoin(t *testing.T) {
+	root := t.TempDir()
+	secret := filepath.Join(filepath.Dir(root), "outside-secret.txt")
+	if err := os.WriteFile(secret, []byte("nope"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = os.Remove(secret) })
+
+	// Even if file exists outside root, traversal must fail.
+	if _, err := safePathUnderRoot(root, ".."+string(os.PathSeparator)+"outside-secret.txt"); err == nil {
+		t.Fatal("expected path escape to be rejected")
+	}
+}
+
+func TestSanitizeFileNameStripsBadChars(t *testing.T) {
+	got := sanitizeFileName(`bad/name<>|?.exe`)
+	if got == "" || got == `bad/name<>|?.exe` {
+		t.Fatalf("sanitize did not clean name: %q", got)
+	}
+}