tools.age-detect: detect gpg-agent identity (priority above yubikey)
When gpg-agent is reachable and the user has a secret encryption
subkey ([E] ssb), emit the age-plugin-gpg default identity
(AGE-PLUGIN-GPG-1H40XPM, the bech32 of an empty body for the gpg
plugin). gpg-agent caches the passphrase / smartcard PIN, so this is
more frictionless than the yubikey path; we therefore place it above
yubikey/solo2 but below se/tpm.