boot: collect garbage weekly and cap boot entries

Run nix.gc weekly to delete generations older than 14 days, and limit
lanzaboote to 10 entries so the ESP menu never lists generations whose
store paths have already been collected.

Assisted-by: Claude:opus-5-5

modules/boot.nix

@@ -26,6 +26,15 @@
   boot.lanzaboote = {
     enable = true;
     pkiBundle = "/var/lib/sbctl";
+    # Keep ESP entries in step with nix.gc so the menu never lists
+    # generations whose store paths were collected.
+    configurationLimit = 10;
+  };
+
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 14d";
   };
 
   boot.loader.efi.canTouchEfiVariables = true;