Give each failure one error value (closes #113)

internal/cli's copies of vault.ErrSecretNotFound, ErrVaultNotFound,
ErrVersionNotFound and ErrSecretExists are removed; the commands wrap the
vault errors. Every error of secret.ReadPassphrase wraps
ErrPassphraseNotRead, so its callers no longer add those words.
ResolveGPGKeyFingerprint returns ErrGPGKeyNotFound for a key the keyring
lacks, recognised by gpg's status line. storeInKeychain returns
errNilDataBuffer. bip85's ErrPasswordTooShort and ErrEncodedTooShort go
with their unreachable checks. Tests that matched these errors' text use
errors.Is.

Model: opus-5-5

internal/cli/create_vault_test.go

@@ -188,7 +188,7 @@ func TestStopAtPassphrasePromptLeavesNothing(t *testing.T) {
 
 			err := tt.run(c)
 
-			require.ErrorContains(t, err, "failed to read passphrase")
+			require.ErrorIs(t, err, secret.ErrPassphraseNotRead)
 			require.Equal(t, before, snapshotStateDir(t, tt.fs))
 		})
 	}