feat(yubikey): configure GPG signing key and disable GCR ssh-agent

Set actual GPG key ID for git commit signing, disable GCR ssh-agent
socket so gpg-agent handles SSH auth, and trim redundant setup steps
from yubikey guide.

yubikey.md

@@ -1,27 +1,5 @@
 # Post-Rebuild YubiKey Setup Guide
 
-## Step 0: Rebuild & Reboot
-
-```bash
-nixos-rebuild switch
-# Then reboot to ensure gpg-agent replaces gnome-keyring as SSH agent
-reboot
-```
-
-After reboot, verify the new environment:
-
-```bash
-gpg --version           # Should show GnuPG 2.4+
-ykman info              # Should show your YubiKey model/serial
-gpg --card-status       # Should show the YubiKey smartcard
-echo $SSH_AUTH_SOCK     # Should point to gpg-agent socket, NOT gcr
-```
-
-> If `gpg --card-status` hangs or errors, restart pcscd:
-> `sudo systemctl restart pcscd`
-
----
-
 ## Step 1: Generate GPG Master Key
 
 ```bash