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.

workstation/snapd.nix

@@ -1,17 +1,10 @@
 { config, lib, ... }:
 
-let
-  # Merge our session variable into any existing ones, if present.
-  existingSessionVars = config.home.sessionVariables or {};
-  sessionVars = existingSessionVars // {
-    # Prepend snap desktop dir so snap-provided .desktop files are discovered by desktop environments.
-    # We include `$XDG_DATA_DIRS` to preserve any previously-set value at runtime.
-    XDG_DATA_DIRS = "/var/lib/snapd/desktop:$XDG_DATA_DIRS";
-  };
-in
 {
   config = {
-    # Use mkForce to ensure this module's value takes precedence when modules are merged.
-    home.sessionVariables = lib.mkForce sessionVars;
+    # Prepend snap desktop dir so snap-provided .desktop files are discovered by desktop environments.
+    # We include `$XDG_DATA_DIRS` to preserve any previously-set value at runtime.
+    # Avoid reading `config.home.sessionVariables` here to prevent evaluation recursion.
+    home.sessionVariables.XDG_DATA_DIRS = "/var/lib/snapd/desktop:$XDG_DATA_DIRS";
   };
 }