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

flake.nix

@@ -0,0 +1,51 @@
+{
+  description = "Various packages for Nix";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    systems.url = "github:nix-systems/default";
+    blueprint = {
+      url = "github:numtide/blueprint";
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.systems.follows = "systems";
+    };
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    flake-parts = {
+      url = "github:hercules-ci/flake-parts";
+      inputs.nixpkgs-lib.follows = "nixpkgs";
+    };
+    bun2nix = {
+      # TODO(#4001): drop the branch pin once catalog support
+      # (nix-community/bun2nix#86) reaches the default branch.
+      url = "github:nix-community/bun2nix/staging-2.1.0";
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.systems.follows = "systems";
+      inputs.treefmt-nix.follows = "treefmt-nix";
+      inputs.flake-parts.follows = "flake-parts";
+    };
+  };
+
+  outputs =
+    inputs:
+    let
+      blueprintOutputs = inputs.blueprint {
+        inherit inputs;
+        nixpkgs.config.allowUnfree = true;
+      };
+
+    in
+    blueprintOutputs
+    // {
+      overlays = {
+        default = import ./overlays {
+          inherit (blueprintOutputs) packages;
+        };
+        shared-nixpkgs = import ./overlays/shared-nixpkgs.nix {
+          inherit (blueprintOutputs) mkPackagesFor;
+        };
+      };
+    };
+}