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 it 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 version 4 OpenPGP
secret key; a protected key's passphrase comes from
MFER_SIGN_KEY_PASSPHRASE or a terminal prompt. gen and freshen check that
the key can sign before they read any file. Verification keeps the rules
of the --require-signature fix: one primary key in the embedded block,
counted from its packets, exactly one signature, made by that key or a
subkey, and signer equal to its fingerprint. The embedded block may hold
no DSA key and no secret key, and an armored field must be one
well-formed block.

Model: opus-5-5

internal/cli/mfer.go

@@ -169,7 +169,7 @@ func requireSignatureFlag() *cli.StringFlag {
 	return &cli.StringFlag{
 		Name:    flagRequireSignature,
 		Aliases: []string{"S"},
-		Usage:   "Require manifest to be signed by the specified GPG key ID",
+		Usage:   "Require manifest to be signed by the OpenPGP key with this fingerprint",
 		Sources: cli.EnvVars("MFER_REQUIRE_SIGNATURE"),
 	}
 }
@@ -229,7 +229,7 @@ func (mfa *CLIApp) generateCommand() *cli.Command {
 			&cli.StringFlag{
 				Name:    "sign-key",
 				Aliases: []string{"s"},
-				Usage:   "GPG key ID to sign the manifest with",
+				Usage:   "OpenPGP secret key file to sign the manifest with",
 				Sources: cli.EnvVars("MFER_SIGN_KEY"),
 			},
 			&cli.StringFlag{
@@ -319,7 +319,7 @@ func (mfa *CLIApp) freshenCommand() *cli.Command {
 			&cli.StringFlag{
 				Name:    "sign-key",
 				Aliases: []string{"s"},
-				Usage:   "GPG key ID to sign the manifest with",
+				Usage:   "OpenPGP secret key file to sign the manifest with",
 				Sources: cli.EnvVars("MFER_SIGN_KEY"),
 			},
 			&cli.BoolFlag{