Initialize Nix overlay repository with blueprint, goose-cli, and docs

Add the complete overlay structure using numtide/blueprint with:
- Two overlay strategies (default and shared-nixpkgs)
- goose-cli package with custom librusty_v8 fetcher
- Interactive package launcher via fzf
- Flake input caching utility
- Comprehensive README and AGENTS.md documentation

overlays/default.nix

@@ -0,0 +1,6 @@
+{
+  packages,
+}:
+final: _prev: {
+  millerson-nix-overlay = packages.${final.stdenv.hostPlatform.system} or { };
+}