fix anti-patterns: niri portals, nix optimise, zram; add dev packages

- niri: remove explicit package override (use niri-flake default), drop
  KDE portal, set xdg.portal.config for deterministic portal resolution,
  remove unused waybar
- core: replace synchronous auto-optimise-store with async
  nix.optimise.automatic, reduce zramSwap.memoryPercent from 300 to 100
- dev: add python3 and uv to system packages
- add .mcp.json (nixos MCP server) and CLAUDE.md

modules/core.nix

@@ -8,7 +8,6 @@
     nix = {
       settings = {
         experimental-features = [ "nix-command" "flakes" ];
-        auto-optimise-store = true;
         substituters = [
           "https://cache.nixos.org"
           "https://niri.cachix.org"
@@ -27,12 +26,13 @@
         dates = "weekly";
         options = "--delete-older-than 7d";
       };
+      optimise.automatic = true;
     };
 
     zramSwap =  {
       enable = true;
       algorithm = "zstd";
-      memoryPercent = 300;
+      memoryPercent = 100;
     };
 
     services.xserver.enable = false;