Upgrade to Nix 25.11 and migrate configs

Update flake inputs and flake.lock to Nix/nixpkgs and home-manager
release 25.11; bump home.stateVersion and setup script NIXOS_RELEASE.

Migrate git config to new Home Manager layout (programs.git.settings,
aliases under settings.alias), adjust delta config path, and reorganize
SSH into matchBlocks with enableDefaultConfig=false to avoid global
leaks. Simplify snapd session variable handling to avoid recursion.

Misc: tweak Dashy title, replace du-dust->dust and plex-media-player ->
plex-desktop, remove unused hostname arg, and add GitHub Copilot
instructions document.

packages/common/packages.nix

@@ -1,6 +1,5 @@
 {
   pkgs,
-  hostname,
   ...
 }:
 {
@@ -31,7 +30,7 @@
     eza # Modern ls
     bat # Modern cat
     broot # Interactive directory navigator
-    du-dust # Modern du
+    dust # Modern du
     duf # Modern df
     zip
     unzip