Remove the keychain and Secure Enclave unlockers (closes #132)

Neither unlocker works without macOS code signing and entitlements, which
secret does not have, so both were research and leave the build.
`secret unlocker add` takes only passphrase and pgp on every platform;
keychain and secure-enclave fail as unknown types and are gone from
--help and shell completion. internal/macse, their files in
internal/secret with their tests, and their branches in internal/vault
and internal/cli are deleted; github.com/keybase/go-keychain leaves
go.mod and go.sum. README and Dockerfile comments no longer mention
them. The research stays at ef0ae90, tagged v1.0.0-rc1.

Judgement call: derivation_index_test.go goes with getLongTermPrivateKey,
its only subject, which only these two unlockers used.
Unverified: not built or run on a Mac; only the GOOS=darwin vet and lint ran.

Model: opus-5-5

internal/secret/atomic_test.go

@@ -12,7 +12,6 @@ import (
 	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	"sneak.berlin/go/secret/internal/macse"
 	"sneak.berlin/go/secret/internal/secret"
 	"sneak.berlin/go/secret/internal/vault"
 )
@@ -900,42 +899,3 @@ func TestWriteDirRefusesExistingDir(t *testing.T) {
 		})
 	}
 }
-
-// TestSecureEnclaveUnlockerFailureDeletesKey makes moving a new Secure
-// Enclave unlocker into place fail after its Secure Enclave key is created:
-// the key must be deleted again. Skipped when the add fails before that, as
-// it does everywhere but in a macOS build with cgo on a Mac with a Secure
-// Enclave.
-func TestSecureEnclaveUnlockerFailureDeletesKey(t *testing.T) {
-	t.Parallel()
-
-	mnemonic := testMnemonicBuffer(t)
-	base := afero.NewMemMapFs()
-	_, err := vault.CreateVault(base, testVaultStateDir, testVaultName, mnemonic, nil)
-	require.NoError(t, err)
-
-	// The unlocker's directory is named se-<label of its Secure Enclave key>
-	var seKeyLabel string
-
-	fs := hookFs{Fs: base, before: func(op, path string) error {
-		if op == opRename && filepath.Base(filepath.Dir(path)) == "unlockers.d" {
-			seKeyLabel = strings.TrimPrefix(filepath.Base(path), "se-")
-
-			return errInjected
-		}
-
-		return nil
-	}}
-
-	_, err = secret.CreateSecureEnclaveUnlocker(fs, testVaultStateDir, mnemonic,
-		nil)
-
-	if seKeyLabel == "" {
-		t.Skipf("the add failed before moving the unlocker into place: %v", err)
-	}
-
-	require.ErrorIs(t, err, errInjected)
-
-	_, err = macse.Encrypt(seKeyLabel, []byte("test"))
-	assert.Error(t, err, "Secure Enclave key left behind")
-}