agent: Support the use case of composite PQC for prompting.
* agent/findkey.c (agent_key_from_file): Take care of the case where GRIP==CTRL->keygrip1. * agent/pkdecrypt.c (composite_pgp_kem_decrypt): Use NULL for the GRIP, it's for crypto operation where prompt is expected.
- GnuPG-bug-id: T7648
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>