refactor: simplify path policy and TUI reducers

internal/pathutil/pathutil.go

@@ -0,0 +1,14 @@
+// Package pathutil contains lexical path-policy helpers. It does not replace
+// descriptor-anchored filesystem access for security-sensitive operations.
+package pathutil
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// Within reports whether path is parent itself or a lexical descendant.
+func Within(path, parent string) bool {
+	relative, err := filepath.Rel(parent, path)
+	return err == nil && relative != ".." && !strings.HasPrefix(relative, ".."+string(filepath.Separator))
+}