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

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "nixos": {
+      "command": "uvx",
+      "args": ["mcp-nixos"]
+    }
+  }
+}