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)

hosts/mreow/default.nix

@@ -8,8 +8,8 @@
 }:
 {
   imports = [
-    ./common.nix
-    ./disk_mreow.nix
+    ../../modules/desktop-common.nix
+    ./disk.nix
 
     inputs.nixos-hardware.nixosModules.framework-amd-ai-300-series
   ];

hosts/mreow/home.nix

@@ -6,12 +6,12 @@
 }:
 {
   imports = [
-    ./gui.nix
-    ./desktop.nix
-    ./progs/borg.nix
+    ../../home/profiles/gui.nix
+    ../../home/profiles/desktop.nix
+    ../../home/progs/borg.nix
 
     # effects headphones too
-    # ./progs/framework-13-easyeffects.nix
+    # ../../home/progs/framework-13-easyeffects.nix
   ];
 
   # media controls