Initial commit: MCP server packages for Nix

- Add flake structure with overlay and packages exports
- Add manim-mcp-server as first packaged MCP server
- Add README.org with usage instructions and package templates
- Add packages/default.nix as package aggregator
- Add overlays/default.nix for nixpkgs integration

overlays/default.nix

@@ -0,0 +1,23 @@
+# MCP Servers Overlay
+#
+# This overlay adds all MCP server packages to nixpkgs.
+#
+# Usage in a flake:
+#   {
+#     inputs.mcp.url = "github:your-user/mcp";
+#
+#     outputs = { nixpkgs, mcp, ... }: {
+#       # Apply the overlay
+#       pkgs = import nixpkgs {
+#         system = "x86_64-linux";
+#         overlays = [ mcp.overlays.default ];
+#       };
+#       # Now pkgs.manim-mcp-server is available
+#     };
+#   }
+#
+final: prev:
+let
+  mcpPackages = import ../packages { pkgs = final; };
+in
+mcpPackages