Add beads Claude plugin installation via home-manager

- Add beadsRepo reference from flake input
- Add activation script to install beads as marketplace plugin
- Updates known_marketplaces.json and config.json declaratively

flake.nix

@@ -42,6 +42,11 @@
       url = "github:Jovian-Experiments/Jovian-NixOS";
       inputs.nixpkgs.follows = "nixpkgs-unstable";
     };
+
+    beads = {
+      url = "github:steveyegge/beads";
+      inputs.nixpkgs.follows = "nixpkgs-unstable";
+    };
   };
 
   outputs = { self, nixpkgs, nixpkgs-unstable, nixos-wsl, ... } @ inputs: let