system: add pipe-to-loki helper script

Adds a system-wide script for sending command output or interactive
sessions to Loki for easy sharing with Claude.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

system/default.nix

@@ -9,6 +9,7 @@
     ./motd.nix
     ./packages.nix
     ./nix.nix
+    ./pipe-to-loki.nix
     ./root-user.nix
     ./pki/root-ca.nix
     ./sshd.nix