phase 3: new flake.nix + extract common-{nix,doas,shell-fish}; rewire imports

- New unified flake with two nixpkgs channels (unstable for desktops, 25.11 for muffin)
- modules/common-{doas,shell-fish,nix}.nix extracted from duplicated blocks
- modules/desktop-common.nix: renamed from system/common.nix; secret paths point to secrets/desktop/
- hosts/{mreow,yarn}/default.nix import desktop-common; yarn imports modules/no-rgb.nix
- hosts/muffin/default.nix imports common-* + server-prefixed modules + services/; duplicate doas/fish/nix blocks removed; gc retention preserved as mkForce override
- modules/age-secrets.nix: file paths → ../secrets/server/*.age
- services/{minecraft,matrix/livekit}: secret paths → ../secrets/server/
- home/profiles/*.nix: ./progs/ → ../progs/
- hosts/{mreow,yarn}/home.nix: imports rewired to ../../home/profiles/ and ../../home/progs/
- home/progs/pi.nix and hosts/yarn/home.nix: secret reads → ../../secrets/home/
- tests/*.nix: ../modules/security.nix → ../modules/server-security.nix; ../modules/overlays.nix → ../lib/overlays.nix
- lib/default.nix: takes explicit lib param (defaults to nixpkgs-stable.lib)

home/profiles/desktop.nix

@@ -7,16 +7,16 @@
 {
   imports = [
     # niri wayland compositor
-    ./progs/niri.nix
+    ../progs/niri.nix
 
     # lockscreen
-    ./progs/swaylock.nix
+    ../progs/swaylock.nix
 
     # notification daemon
-    ./progs/dunst.nix
+    ../progs/dunst.nix
 
     # noctalia desktop shell
-    ./progs/noctalia.nix
+    ../progs/noctalia.nix
   ];
 
   home.packages = with pkgs; [

home/profiles/gui.nix

@@ -8,12 +8,12 @@
 {
   imports = [
     ./no-gui.nix
-    # ./progs/ghostty.nix
-    ./progs/alacritty.nix
-    ./progs/emacs.nix
-    # ./progs/trezor.nix # - broken
-    ./progs/flatpak.nix
-    ./progs/zen
+    # ../progs/ghostty.nix
+    ../progs/alacritty.nix
+    ../progs/emacs.nix
+    # ../progs/trezor.nix # - broken
+    ../progs/flatpak.nix
+    ../progs/zen
   ];
 
   nixpkgs.config.allowUnfreePredicate =

home/profiles/no-gui.nix

@@ -117,9 +117,9 @@ let
 in
 {
   imports = [
-    ./progs/fish.nix
-    ./progs/helix.nix
-    ./progs/pi.nix
+    ../progs/fish.nix
+    ../progs/helix.nix
+    ../progs/pi.nix
     (
       { ... }:
       {