refactor: remove dead code, extract shared agent options, optimize flake

- Remove dead overlays/default.nix (flake defines overlays inline)
- Remove orphaned overlays/mods/{beads,n8n}.nix (never imported)
- Remove docs/packages/notesmd-cli.md (package doesn't exist)
- Extract externalSkills submodule to shared-options.nix (eliminates
  ~100 lines of duplication across opencode/claude-code/pi modules)
- Fix lib output: use nixpkgs.lib directly instead of instantiating
  a full nixpkgs just to get lib
- Add lib unit tests to flake checks
- Update stale comment in coding-rules.nix

overlays/default.nix

@@ -1,24 +0,0 @@
-{inputs, ...}: {
-  # This one brings our custom packages from the 'pkgs' directory
-  additions = final: prev: (import ../pkgs {pkgs = final;});
-
-  # This one contains whatever you want to overlay
-  # You can change versions, add patches, set compilation flags, anything really.
-  # https://nixos.wiki/wiki/Overlays
-  modifications = final: prev:
-  # Import all package modifications from mods directory
-    (import ./mods/default.nix {inherit prev;})
-    // {
-      # Direct configuration overrides
-      brave = prev.brave.override {
-        commandLineArgs = "--password-store=gnome-libsecret";
-      };
-    };
-
-  master-packages = final: _prev: {
-    master = import inputs.nixpkgs-master {
-      system = final.stdenv.hostPlatform.system;
-      config.allowUnfree = true;
-    };
-  };
-}