build: develop in a nix shell, and lint each commit

.envrc

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

.github/workflows/ci.yml

@@ -25,4 +25,4 @@ jobs:
           bun-version: "1.4.2"
       - run: bun install --frozen-lockfile
       - run: bun run check
-      - run: npm pack --dry-run
+      - run: bun pm pack --dry-run

.gitignore

@@ -7,3 +7,5 @@ coverage/
 .DS_Store
 *.log
 .claude/
+.direnv/
+.pre-commit-config.yaml

.zed/settings.json

@@ -0,0 +1,45 @@
+{
+  "format_on_save": "on",
+  "lsp": {
+    "biome": {
+      "settings": {
+        "require_config_file": true
+      }
+    },
+    "nil": {
+      "binary": {
+        "path": "direnv",
+        "arguments": ["exec", ".", "nil"]
+      }
+    }
+  },
+  "languages": {
+    "JavaScript": {
+      "formatter": { "language_server": { "name": "biome" } },
+      "code_actions_on_format": {
+        "source.organizeImports.biome": true
+      }
+    },
+    "TypeScript": {
+      "formatter": { "language_server": { "name": "biome" } },
+      "code_actions_on_format": {
+        "source.organizeImports.biome": true
+      }
+    },
+    "JSON": {
+      "formatter": { "language_server": { "name": "biome" } }
+    },
+    "JSONC": {
+      "formatter": { "language_server": { "name": "biome" } }
+    },
+    "Nix": {
+      "language_servers": ["nil", "!nixd"],
+      "formatter": {
+        "external": {
+          "command": "direnv",
+          "arguments": ["exec", ".", "nixfmt", "-"]
+        }
+      }
+    }
+  }
+}

README.md

@@ -147,7 +147,9 @@ gets**.
 
 ## Development
 
-You need Bun 1.4.2, Python 3 and macOS or Linux.
+You need Bun 1.4.2, Python 3 and macOS or Linux. [Nix](https://nixos.org)
+gives the shell all three, and lints each commit with Biome and nixfmt:
+`nix develop`, or `direnv allow` once.
 
 ```sh
 bun install --frozen-lockfile

flake.nix

@@ -0,0 +1,69 @@
+{
+  description = "The Leverage plugin for Pi: its development shell";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
+    # Only for Bun: stable has 1.3.13, and the project pins 1.4.2.
+    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
+    git-hooks.url = "github:cachix/git-hooks.nix";
+    git-hooks.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      nixpkgs-unstable,
+      git-hooks,
+      ...
+    }:
+    let
+      systems = [
+        "aarch64-darwin"
+        "x86_64-darwin"
+        "aarch64-linux"
+        "x86_64-linux"
+      ];
+      forEach = f: nixpkgs.lib.genAttrs systems (system: f system nixpkgs.legacyPackages.${system});
+      # Lints each commit once the shell is entered.
+      hooks = forEach (
+        system: pkgs:
+        git-hooks.lib.${system}.run {
+          src = ./.;
+          hooks = {
+            # The project's own Biome, the version CI checks with.
+            biome = {
+              enable = true;
+              name = "biome";
+              entry = "node_modules/.bin/biome check --write --no-errors-on-unmatched --files-ignore-unknown=true";
+              types_or = [
+                "javascript"
+                "jsx"
+                "ts"
+                "tsx"
+                "json"
+              ];
+            };
+            nixfmt.enable = true;
+          };
+        }
+      );
+    in
+    {
+      devShells = forEach (
+        system: pkgs: {
+          default = pkgs.mkShell {
+            inherit (hooks.${system}) shellHook;
+            packages = [
+              nixpkgs-unstable.legacyPackages.${system}.bun
+              pkgs.nodejs_24
+              # The tests drive real terminals through Python.
+              pkgs.python3
+              # Zed's language server for flake.nix.
+              pkgs.nil
+            ]
+            ++ hooks.${system}.enabledPackages;
+          };
+        }
+      );
+    };
+}