nix: enable auto-optimise-store

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common.nix

@@ -116,6 +116,10 @@
       busybox
   ];
 
+  # Hard-link identical files in the store as new paths are added, so the
+  # store de-duplicates itself on every build instead of drifting.
+  nix.settings.auto-optimise-store = true;
+
   # Offload builds to the media server. Excluded on the server itself to
   # avoid a pointless SSH round-trip to localhost.
   nix.distributedBuilds =