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-nix.nix

@@ -0,0 +1,22 @@
+{ lib, ... }:
+{
+  # Common Nix daemon settings. Host-specific overrides (binary cache substituters,
+  # gc retention) live in the host's default.nix.
+  nix = {
+    optimise.automatic = true;
+
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      # Default retention: override per-host via lib.mkForce if different.
+      options = lib.mkDefault "--delete-older-than 30d";
+    };
+
+    settings = {
+      experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+    };
+  };
+}