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/fetch.go

@@ -455,7 +455,6 @@ func (mfa *CLIApp) fetchManifest(
 	}
 
 	// Parse manifest
-	//nolint:contextcheck // mfer loads a manifest without a context
 	manifest, err := mfer.NewManifestFromReader(bytes.NewReader(manifestData))
 	if err != nil {
 		return nil, nil, fmt.Errorf("parse manifest: %w", err)
@@ -463,7 +462,6 @@ func (mfa *CLIApp) fetchManifest(
 
 	requiredSigner := cmd.String(flagRequireSignature)
 	if requiredSigner != "" {
-		//nolint:contextcheck // mfer loads a manifest without a context
 		err = verifyFetchedSigner(manifestData, requiredSigner)
 		if err != nil {
 			return nil, nil, err