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.

config/ssh.nix

@@ -3,14 +3,20 @@
 {
   programs.ssh = {
     enable = true;
-    compression = true;
-    serverAliveInterval = 60;
-    serverAliveCountMax = 3;
+    enableDefaultConfig = false;
 
-    # SSH Multiplexing - reuses existing SSH connections for multiple sessions, reducing authentication overhead and improving speed for subsequent logins.
-    controlPath = "~/.ssh/master-%r@%n:%p";
-    controlMaster = "auto";
-    controlPersist = "600";
+    matchBlocks = {
+      "*" = {
+        compression = true;
+        serverAliveInterval = 60;
+        serverAliveCountMax = 3;
+
+        # SSH Multiplexing - reuses existing SSH connections for multiple sessions, reducing authentication overhead and improving speed for subsequent logins.
+        controlPath = "~/.ssh/master-%r@%n:%p";
+        controlMaster = "auto";
+        controlPersist = "600";
+      };
+    };
 
     # Include custom configs from 1Password (See packages/common/secrets.nix)
     includes = [