Sign and verify manifests in Go with OpenPGP instead of running gpg (closes #181)

mfer ran the gpg binary to sign, export keys and verify, so signing and
loading signed manifests failed wherever gpg is missing. It now uses
github.com/ProtonMail/go-crypto/openpgp. --sign-key and MFER_SIGN_KEY
name a file holding one OpenPGP secret key; a protected key's passphrase
comes from MFER_SIGN_KEY_PASSPHRASE or a prompt on the terminal, and is
checked before any file is read. Verification keeps the rules of the
--require-signature fix: one primary key in the embedded block, counted
from its packets so that keys the library skips count too, exactly one
signature, made by that key or one of its subkeys, and signer equal to
its fingerprint. An armored key or signature must be one block and
nothing else.

Model: opus-5-5

internal/cli/export.go

@@ -36,7 +36,6 @@ func (mfa *CLIApp) exportManifestOperation(
 
 	defer func() { _ = rc.Close() }()
 
-	//nolint:contextcheck // mfer loads a manifest without a context
 	manifest, err := mfer.NewManifestFromReader(rc)
 	if err != nil {
 		return fmt.Errorf("parse manifest: %w", err)