Merge remote; add nix auto-GC and CLAUDE.md

Resolves conflict with upstream tmux/blender additions (already covered
by the per-program restructure). Adds weekly nix GC (--delete-older-than 7d),
store auto-optimise, and project CLAUDE.md.

system/nix.nix

@@ -3,4 +3,11 @@
 {
   nix.settings.experimental-features = [ "nix-command" "flakes" ];
   nixpkgs.config.allowUnfree = true;
+
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 7d";
+  };
+  nix.settings.auto-optimise-store = true;
 }