Switch to ncps.

common/global/nix.nix

@@ -6,15 +6,14 @@
     ];
 
     # Binary cache configuration
+    # c3 runs ncps (Nix Cache Proxy Server) that caches cache.nixos.org
+    # Falls back to cache.nixos.org if c3 is unreachable
     substituters = [
-      "http://c3:5000"  # Local cluster cache on c3
+      "http://c3:8501"  # Local ncps cache proxy on c3
       "https://cache.nixos.org"
     ];
     trusted-public-keys = [
       "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
-      # TODO: Add c3 cache public key here after first deploy of c3
-      # Get it with: ssh c3 cat /persist/nix-cache/cache-pub-key.txt
-      # "c3-cache:..."
     ];
 
     # Performance tuning