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)

modules/common-doas.nix

@@ -0,0 +1,15 @@
+{ username, ... }:
+{
+  # doas replaces sudo on every host
+  security = {
+    doas.enable = true;
+    sudo.enable = false;
+    doas.extraRules = [
+      {
+        users = [ username ];
+        keepEnv = true;
+        persist = true;
+      }
+    ];
+  };
+}