nixpkgs: Refresh channels and inputs

Rebase the fork branches and refresh nixpkgs, home-manager, and the
approved ancillary inputs. Update kernel and release metadata, adapt
removed package and Home Assistant options, and keep Determinate Nix
on its tested nixpkgs revision to avoid duplicate Boost patches.

Retire Sharry and its public endpoint because copyparty replaces it.
Document the GitHub mirror gate and require real devshell and system
builds in the upgrade validation workflow.

lib/constants.nix

@@ -30,7 +30,7 @@ rec {
 
   kernel = {
     lts = pkgs: pkgs.linuxKernel.packages.linux_6_18;
-    latest = pkgs: pkgs.linuxKernel.packages.linux_7_0;
+    latest = pkgs: pkgs.linuxKernel.packages.linux_7_2;
   };
 
   nginx = rec {