ci: add a shared ESLint flat-config lint gate

Add eslint.config.mjs (eslint:recommended + typescript-eslint recommended,
type-unaware) with a `lint` script and a CI step. Rules that need a baseline
cleanup pass (no-explicit-any, ban-ts-comment, no-useless-assignment,
preserve-caught-error) are warnings; core rules and unused vars are errors.

Refs Heretek-Games/drop#69 (audit finding F-23).

.github/workflows/ci.yml

@@ -30,3 +30,5 @@ jobs:
 
       - name: Test
         run: npm test
+      - name: Lint
+        run: npm run lint

eslint.config.mjs

@@ -0,0 +1,35 @@
+// @ts-check
+import eslint from "@eslint/js";
+import tseslint from "typescript-eslint";
+
+// Shared lint gate for the Heretek Games Drop plugins. Intentionally not
+// type-aware (fast, no tsconfig program). Rules that would require a
+// pre-existing cleanup pass are warnings so the gate lands green; core rules
+// and unused vars remain errors.
+export default tseslint.config(
+  {
+    ignores: [
+      "**/dist/**",
+      "**/node_modules/**",
+      "plugin-bundle/**",
+      "dist-packages/**",
+    ],
+  },
+  eslint.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    rules: {
+      // TypeScript already reports undeclared identifiers; no-undef only adds
+      // false positives for runtime globals (fetch, process, ...).
+      "no-undef": "off",
+      "no-useless-assignment": "warn",
+      "preserve-caught-error": "warn",
+      "@typescript-eslint/no-explicit-any": "warn",
+      "@typescript-eslint/ban-ts-comment": "warn",
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
+      ],
+    },
+  },
+);

package.json

@@ -8,7 +8,8 @@
     "build": "rm -rf dist && tsc",
     "test": "node --test dist/test/*.js",
     "typecheck": "tsc --noEmit",
-    "sign": "drop-plugin sign ."
+    "sign": "drop-plugin sign .",
+    "lint": "eslint ."
   },
   "dependencies": {
     "@droposs/plugin-sdk": "^0.4.0"
@@ -16,7 +17,10 @@
   "devDependencies": {
     "@droposs/plugin-cli": "^0.4.0",
     "typescript": "^5.7.0",
-    "@types/node": "^22.0.0"
+    "@types/node": "^22.0.0",
+    "@eslint/js": "^10.0.1",
+    "eslint": "^10.9.1",
+    "typescript-eslint": "^8.68.0"
   },
   "license": "MIT"
-}
+}