Decouple checkpoint storage from workspace state

internal/core/config/cache.go

@@ -0,0 +1,23 @@
+package config
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+// DefaultChunkPlanRoot resolves the existing per-user chunk-plan cache root.
+func DefaultChunkPlanRoot(userCacheDir func() (string, error)) (string, error) {
+	if userCacheDir == nil {
+		return "", fmt.Errorf("user cache directory resolver must not be nil")
+	}
+	root, err := userCacheDir()
+	if err != nil {
+		return "", fmt.Errorf("resolve user cache directory: %w", err)
+	}
+	root = strings.TrimSpace(root)
+	if root == "" {
+		return "", fmt.Errorf("user cache directory must not be empty")
+	}
+	return filepath.Join(filepath.Clean(root), "notarius", "chunk-plans"), nil
+}