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.

setup.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 IFS=$'\n\t'
 
 # Constants
-readonly NIXOS_RELEASE="25.05"                                      # Home Manager release version (Must match NixOS version)
+readonly NIXOS_RELEASE="25.11"                                      # Home Manager release version (Must match NixOS version)
 readonly GIT_REPO="https://git.mvl.sh/vleeuwenmenno/dotfiles.git"   # Dotfiles repository URL
 readonly DOTFILES_PATH="${HOME}/.dotfiles"                          # Dotfiles directory
 readonly SETUP_MARKER="${HOME}/.dotfiles-setup"                     # Setup marker file indicates setup has been run