Fix identity encryption always using piv-p256 instead of p256tag/mlkem768p256tag

[GaMoCh]

Summary

• When encrypting with an identity (-i), the plugin always used piv-p256 stanza type, ignoring post-quantum keys
• Now uses mlkem768p256tag when the identity has ML-KEM keys, and p256tag otherwise
• Aligns with age-plugin-yubikey behavior of always using modern stanza types for encryption

Before / After

Before

After

Details

The runRecipientV1 function hardcoded .pivp256 for all identities passed via
add-identity, even when the identity contained ML-KEM-768 keys generated with --pq.
This meant age -e -i key.txt never used post-quantum encryption.

Recipients passed via add-recipient (-r) already correctly detected the stanza type
from the recipient string prefix. This fix applies equivalent logic for identities,
using the available key material to determine the appropriate stanza type.