podman: Enable daily auto-prune with --all flag

Without --all, only dangling (untagged) images were pruned,
allowing tagged-but-unused CI images to accumulate (~223G).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/base.nix

@@ -84,6 +84,8 @@ makeEnable config "myModules.base" true {
     enable = true;
     autoPrune = {
       enable = true;
+      dates = "daily";
+      flags = ["--all"];
     };
   };