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_test.go

@@ -36,7 +36,6 @@ package secret_test
 
 import (
 	"errors"
-	"fmt"
 	"path/filepath"
 	"testing"
 	"time"
@@ -127,27 +126,6 @@ func TestGenerateVersionName(t *testing.T) {
 	assert.NotEqual(t, version1, version2)
 }
 
-func TestGenerateVersionNameMaxSerial(t *testing.T) {
-	t.Parallel()
-
-	fs := afero.NewMemMapFs()
-	secretDir := testSecretDir
-	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 := secret.GenerateVersionName(fs, secretDir)
-	require.Error(t, err)
-	assert.Contains(t, err.Error(), "exceeded maximum versions per day")
-}
-
 func TestNewVersion(t *testing.T) {
 	t.Parallel()