Refuse templates read through a symlink anywhere under files/

A symlinked files/ or a symlinked directory under it let module check, the merge
gate and module add read a template from outside the repository tree, and a
symlink no template names passed them while the build's packing refuses it
(review of mesh-controller#313). Every entry under files/ must now be a plain
directory or file, and templates are read through an os.Root of the module's
directory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/catalogue/template_test.go

@@ -256,3 +256,68 @@ func TestAManifestStillNamingATemplateIsNotHeld(t *testing.T) {
 		t.Fatalf("an inline manifest is refused: %s", problem)
 	}
 }
+
+// A symlinked files/, or a symlinked directory under it, would read a file outside the tree: refused by name,
+// as is any symlink or special file under files/ that no template names, which the build's packing refuses.
+func TestASymlinkAnywhereUnderFilesIsRefusedByName(t *testing.T) {
+	m, err := ParseManifest([]byte(templatedTwin))
+	if err != nil {
+		t.Fatal(err)
+	}
+	outside := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(outside, "deeper"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	for _, name := range []string{"tidy.conf", "deeper/extra.conf", "secret"} {
+		if err := os.WriteFile(filepath.Join(outside, filepath.FromSlash(name)), []byte("outside"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	write := func(t *testing.T, dir, name string) {
+		t.Helper()
+		full := filepath.Join(dir, filepath.FromSlash(name))
+		if err := os.MkdirAll(filepath.Dir(full), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(full, []byte("inside"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	link := func(t *testing.T, target, at string) {
+		t.Helper()
+		if err := os.MkdirAll(filepath.Dir(at), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Symlink(target, at); err != nil {
+			t.Fatal(err)
+		}
+	}
+	for _, c := range []struct {
+		name  string
+		make  func(t *testing.T, dir string)
+		names string
+	}{
+		{"files/ itself", func(t *testing.T, dir string) {
+			link(t, outside, filepath.Join(dir, "files"))
+		}, "files/ is a symlink"},
+		{"a directory under files/", func(t *testing.T, dir string) {
+			write(t, dir, "files/tidy.conf")
+			link(t, filepath.Join(outside, "deeper"), filepath.Join(dir, "files", "deeper"))
+		}, "files/deeper is a symlink"},
+		{"a file no template names", func(t *testing.T, dir string) {
+			write(t, dir, "files/tidy.conf")
+			write(t, dir, "files/deeper/extra.conf")
+			link(t, filepath.Join(outside, "secret"), filepath.Join(dir, "files", "unnamed"))
+		}, "files/unnamed is a symlink"},
+	} {
+		dir := t.TempDir()
+		c.make(t, dir)
+		_, err := WithTemplatesFrom(m, dir)
+		if err == nil || !strings.Contains(err.Error(), c.names) {
+			t.Errorf("a symlinked %s was not refused by name: %v", c.name, err)
+		}
+		if err != nil && strings.Contains(err.Error(), "outside") {
+			t.Errorf("a symlinked %s was read: %v", c.name, err)
+		}
+	}
+}