Add tests for internal/storage: URL parsing, backends, shared conformance suite (closes #66)

internal/storage, the package that parses store URLs and selects the backend, had no tests.

Adds table-driven tests for URL parsing (each scheme, query parameters, malformed input, unknown scheme, backend type chosen); one shared conformance suite for the Storer interface, run against the file backend in a temp directory and the s3 backend on the in-process harness internal/s3 already uses, so a new backend inherits it; and rclone construction and argument tests using its in-process local backend. A comment records that rclone data operations need a configured remote and are not unit-tested. No production code changed and no defect surfaced.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

internal/storage/file_backend_test.go

@@ -0,0 +1,27 @@
+package storage_test
+
+import (
+	"testing"
+
+	"sneak.berlin/go/vaultik/internal/storage"
+)
+
+// newFileStorer builds a file:// backend rooted at a fresh temp directory.
+//
+//nolint:ireturn // conformance runs against the Storer interface by design
+func newFileStorer(t *testing.T) storage.Storer {
+	t.Helper()
+
+	s, err := storage.NewFileStorer(t.TempDir())
+	if err != nil {
+		t.Fatalf("NewFileStorer: %v", err)
+	}
+
+	return s
+}
+
+// TestFileStorer runs the shared Storer contract against the file:// backend.
+func TestFileStorer(t *testing.T) {
+	t.Parallel()
+	runStorerConformance(t, newFileStorer)
+}