Add Nix flake for Emacs development environment

flake.nix

@@ -0,0 +1,61 @@
+{
+  description = "Emacs mode development environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    jailed-agents.url = "path:/conf/jailed-agents";
+  };
+
+  outputs =
+    { self, nixpkgs, flake-utils, jailed-agents, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+
+        # Choose your Emacs build (e.g., pkgs.emacs, pkgs.emacs-nox, pkgs.emacs-pgtk)
+        epkgs = pkgs.emacsPackagesFor pkgs.emacs;
+
+        # Emacs wrapped with useful dev/test/lint packages available in load-path
+        emacsWithDeps = epkgs.emacsWithPackages (p: with p; [
+          package-lint
+          relint
+          buttercup
+        ]);
+
+        devInputs = with pkgs; [
+          emacsWithDeps
+          epkgs.eldev
+          git
+          gnumake
+          texinfo
+          ripgrep
+        ];
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = devInputs ++ [
+            (jailed-agents.lib.${system}.makeJailedOpencode system {
+              extraPkgs = devInputs;
+            })
+          ];
+
+          shellHook = ''
+            echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+            echo "  Emacs Mode Development Environment"
+            echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+            echo ""
+            echo "Common tasks:"
+            echo "  eldev lint        # package-lint + relint"
+            echo "  eldev test        # run ERT/Buttercup tests"
+            echo "  eldev compile     # byte-compile"
+            echo "  eldev package     # build release tarball"
+            echo ""
+            echo "Start Emacs (preconfigured load-path):"
+            echo "  emacs ."
+            echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+          '';
+        };
+      }
+    );
+}