Apple's built-in ssh-agent has no sk-api/libfido2 support and refuses
signing operations for ed25519-sk / ecdsa-sk hardware keys. Enable the
existing metacfg.security.ssh module (which runs pkgs.openssh's
ssh-agent under launchd) via the common darwin suite, and export
SSH_AUTH_SOCK from environment.shellInit so bash, zsh, and fish (via
/etc/fish/foreign-env/shellInit) all point at the nix-managed socket.
- Commented out default SSH service enablement in common suite.
- Updated SSH agent configuration to use dynamic user paths.
- Removed outdated SSH keys and added missing key descriptions.
