dev: add flake.nix and .envrc for Rust dev environment

- rust-overlay for stable toolchain + rust-src + rust-analyzer
- pkg-config, openssl, sqlite for reqwest + ownership DB
- cargo-watch, cargo-nextest for dev workflow
- direnv integration via .envrc

.envrc

@@ -0,0 +1 @@
+use flake

flake.nix

@@ -0,0 +1,39 @@
+{
+  description = "idx-cli — CLI tool for Indonesian stock market (IDX) analysis";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+        rustToolchain = pkgs.rust-bin.stable.latest.default.override {
+          extensions = [ "rust-src" "rust-analyzer" ];
+        };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            rustToolchain
+            pkg-config
+            openssl
+            sqlite
+            cargo-watch
+            cargo-nextest
+          ];
+
+          env = {
+            RUST_BACKTRACE = "1";
+          };
+        };
+      }
+    );
+}