Check errors by identity, not by message text, in tests (closes #49)

Tests that asserted a failure by a fragment of its message now use
errors.Is: a refactor returning the wrong error, or wrapping with %v
instead of %w, now fails them. New tests return each exported error of
internal/vault and pkg/bip85 that no test returned, and check wrapped
causes (os.ErrNotExist, ErrMnemonicMismatch through GetSecret,
ErrInvalidPathComponent through DeriveBIP85Entropy). The 999-versions
test moves into package secret to name its unexported error. Checks of
errors no test can name keep their text; they are listed on the issue.

Model: opus-5-5

internal/secret/version_internal_test.go

@@ -0,0 +1,31 @@
+package secret
+
+import (
+	"fmt"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerateVersionNameMaxSerial(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	secretDir := "/test/secret"
+	versionsDir := filepath.Join(secretDir, "versions")
+
+	// Create 999 versions
+	today := time.Now().Format("20060102")
+	for i := 1; i <= 999; i++ {
+		versionName := fmt.Sprintf("%s.%03d", today, i)
+		err := fs.MkdirAll(filepath.Join(versionsDir, versionName), 0o755)
+		require.NoError(t, err)
+	}
+
+	// Try to create one more - should fail
+	_, err := GenerateVersionName(fs, secretDir)
+	require.ErrorIs(t, err, errMaxVersionsPerDay)
+}