feat: scaffold codespace backend

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

internal/util/path.go

@@ -0,0 +1,34 @@
+package util
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// CleanRelative validates and cleans a workspace-relative path.
+// It rejects absolute paths and paths that escape the workspace root via "..".
+// Empty path and "." return ".".
+func CleanRelative(path string) (string, error) {
+	if path == "" || path == "." {
+		return ".", nil
+	}
+
+	// Reject absolute paths.
+	if filepath.IsAbs(path) {
+		return "", New(CodeBadRequest, "absolute paths are not allowed")
+	}
+
+	cleaned := filepath.Clean(path)
+
+	// Reject paths that escape the workspace root.
+	if cleaned == ".." || strings.HasPrefix(cleaned, ".."+string(filepath.Separator)) {
+		return "", New(CodeBadRequest, "path escapes workspace root")
+	}
+
+	return cleaned, nil
+}
+
+// JoinClean joins base and elem after cleaning, returning a cleaned path.
+func JoinClean(base, elem string) string {
+	return filepath.Clean(filepath.Join(base, elem))
+}