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-shell-fish.nix

@@ -0,0 +1,16 @@
+{ pkgs, lib, ... }:
+{
+  # https://nixos.wiki/wiki/Fish#Setting_fish_as_your_shell
+  # Login shells stay bash but immediately `exec fish` so fish is the effective shell
+  # without breaking scripts that hardcode #!/bin/bash.
+  programs.fish.enable = true;
+  programs.bash = {
+    interactiveShellInit = ''
+      if [[ $(${pkgs.procps}/bin/ps --no-header --pid=$PPID --format=comm) != "fish" && -z ''${BASH_EXECUTION_STRING} ]]
+      then
+        shopt -q login_shell && LOGIN_OPTION='--login' || LOGIN_OPTION=""
+        exec ${lib.getExe pkgs.fish} $LOGIN_OPTION
+      fi
+    '';
+  };
+}