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/vault/vault_error_test.go

@@ -1,6 +1,7 @@
 package vault_test
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -36,8 +37,8 @@ func TestAddSecretFailsWithMissingPublicKey(t *testing.T) {
 	defer value.Destroy()
 
 	err := vlt.AddSecret(testSecretName, value, false)
-	require.Error(t, err, "AddSecret should fail when public key is missing")
-	assert.Contains(t, err.Error(), "failed to read long-term public key")
+	require.ErrorIs(t, err, os.ErrNotExist,
+		"AddSecret should fail when public key is missing")
 
 	// Verify that the secret directory was NOT created
 	secretDir := filepath.Join(vaultDir, "secrets.d", testSecretName)