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/shared-nixpkgs.nix

@@ -0,0 +1,10 @@
+{
+  mkPackagesFor,
+}:
+# Builds the packages/ tree against the consumer's `final`, so deps are
+# shared with the rest of their system and no second nixpkgs is
+# evaluated. Trade-off vs overlays.default: the binary cache only hits
+# when the consumer's nixpkgs revision matches ours.
+final: _prev: {
+  millerson-nix-overlay = mkPackagesFor final;
+}