Migrate ESLint to flat config for v9 compatibility

Replaces the FlatCompat shim (which loaded .eslintrc.cjs via @eslint/eslintrc
and emitted deprecation warnings) with a native ESLint 9 flat config. Adds
typescript-eslint meta-package as the flat-config entrypoint and pins
@eslint/js to v9 to satisfy the eslint@9 peer range. Rule set is preserved
verbatim from the Phase 1A-1 plan (Task 4).

.eslintignore

@@ -1,6 +0,0 @@
-dist/
-node_modules/
-ClientApp/
-wwwroot/
-*.cjs
-pnpm-lock.yaml

.eslintrc.cjs

@@ -1,34 +0,0 @@
-/** @type {import('eslint').Linter.Config} */
-module.exports = {
-  root: true,
-  parser: "@typescript-eslint/parser",
-  parserOptions: {
-    ecmaVersion: 2023,
-    sourceType: "module",
-    project: "./tsconfig.json",
-    ecmaFeatures: { jsx: true },
-  },
-  plugins: ["@typescript-eslint", "unused-imports"],
-  extends: [
-    "eslint:recommended",
-    "plugin:@typescript-eslint/recommended",
-  ],
-  rules: {
-    "@typescript-eslint/no-unused-vars": "off",
-    "unused-imports/no-unused-imports": "error",
-    "unused-imports/no-unused-vars": [
-      "warn",
-      {
-        vars: "all",
-        varsIgnorePattern: "^_",
-        args: "after-used",
-        argsIgnorePattern: "^_",
-      },
-    ],
-    "@typescript-eslint/consistent-type-imports": ["error", { prefer: "type-imports" }],
-    "@typescript-eslint/no-explicit-any": "warn",
-    "@typescript-eslint/no-non-null-assertion": "warn",
-    "no-console": ["warn", { allow: ["warn", "error"] }],
-  },
-  ignorePatterns: ["dist/", "node_modules/", "ClientApp/", "wwwroot/", "*.cjs"],
-};

eslint.config.js

@@ -1,37 +1,55 @@
-// ESLint v9 flat-config shim that loads the legacy `.eslintrc.cjs` via FlatCompat.
+// ESLint v9 flat config. Mirrors the rule set from
-// The plan (`docs/superpowers/plans/2026-04-14-phase-1a1-skeleton.md`, Task 4)
+// docs/superpowers/plans/2026-04-14-phase-1a1-skeleton.md (Task 4).
-// specifies `.eslintrc.cjs` verbatim, but ESLint v9 requires a flat config file
-// to be present. This shim preserves the plan's rule set without modifying
-// package.json's `lint` script. A future batch can migrate fully to flat config.
-
-import { FlatCompat } from "@eslint/eslintrc";
 import js from "@eslint/js";
-import { createRequire } from "node:module";
+import tseslint from "typescript-eslint";
-import { fileURLToPath } from "node:url";
+import unusedImports from "eslint-plugin-unused-imports";
-import path from "node:path";
-
-const require = createRequire(import.meta.url);
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = path.dirname(__filename);
-
-const legacyConfig = require("./.eslintrc.cjs");
-
-const compat = new FlatCompat({
-  baseDirectory: __dirname,
-  resolvePluginsRelativeTo: __dirname,
-  recommendedConfig: js.configs.recommended,
-});
 
 export default [
   {
     ignores: [
-      "dist/",
+      "dist/**",
-      "node_modules/",
+      "node_modules/**",
-      "ClientApp/",
+      "ClientApp/**",
-      "wwwroot/",
+      "wwwroot/**",
       "**/*.cjs",
       "pnpm-lock.yaml",
     ],
   },
-  ...compat.config(legacyConfig),
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    files: ["src/**/*.{ts,tsx}"],
+    languageOptions: {
+      parser: tseslint.parser,
+      parserOptions: {
+        ecmaVersion: 2023,
+        sourceType: "module",
+        project: "./tsconfig.json",
+        ecmaFeatures: { jsx: true },
+      },
+    },
+    plugins: {
+      "unused-imports": unusedImports,
+    },
+    rules: {
+      "@typescript-eslint/no-unused-vars": "off",
+      "unused-imports/no-unused-imports": "error",
+      "unused-imports/no-unused-vars": [
+        "warn",
+        {
+          vars: "all",
+          varsIgnorePattern: "^_",
+          args: "after-used",
+          argsIgnorePattern: "^_",
+        },
+      ],
+      "@typescript-eslint/consistent-type-imports": [
+        "error",
+        { prefer: "type-imports" },
+      ],
+      "@typescript-eslint/no-explicit-any": "warn",
+      "@typescript-eslint/no-non-null-assertion": "warn",
+      "no-console": ["warn", { allow: ["warn", "error"] }],
+    },
+  },
 ];

package.json

@@ -24,6 +24,7 @@
     "zod": "^3.23.0"
   },
   "devDependencies": {
+    "@eslint/js": "^9.39.4",
     "@types/node": "^24.0.0",
     "@typescript-eslint/eslint-plugin": "^8.0.0",
     "@typescript-eslint/parser": "^8.0.0",
@@ -31,6 +32,7 @@
     "eslint": "^9.0.0",
     "eslint-plugin-unused-imports": "^4.0.0",
     "typescript": "^5.5.0",
+    "typescript-eslint": "^8.58.2",
     "vite": "^6.0.0",
     "vitest": "^3.0.0"
   }

pnpm-lock.yaml

@@ -12,6 +12,9 @@ importers:
         specifier: ^3.23.0
         version: 3.25.76
     devDependencies:
+      '@eslint/js':
+        specifier: ^9.39.4
+        version: 9.39.4
       '@types/node':
         specifier: ^24.0.0
         version: 24.12.2
@@ -33,6 +36,9 @@ importers:
       typescript:
         specifier: ^5.5.0
         version: 5.9.3
+      typescript-eslint:
+        specifier: ^8.58.2
+        version: 8.58.2(eslint@9.39.4)(typescript@5.9.3)
       vite:
         specifier: ^6.0.0
         version: 6.4.2(@types/node@24.12.2)
@@ -927,6 +933,13 @@ packages:
     resolution: {integrity: sha512-XleUoc9uwGXqjWwXaUTZAmzMcFZ5858QA2vvx1Ur5xIcixXIP+8LnFDgRplU30us6teqdlskFfu+ae4K79Ooew==}
     engines: {node: '>= 0.8.0'}
 
+  typescript-eslint@8.58.2:
+    resolution: {integrity: sha512-V8iSng9mRbdZjl54VJ9NKr6ZB+dW0J3TzRXRGcSbLIej9jV86ZRtlYeTKDR/QLxXykocJ5icNzbsl2+5TzIvcQ==}
+    engines: {node: ^18.18.0 || ^20.9.0 || >=21.1.0}
+    peerDependencies:
+      eslint: ^8.57.0 || ^9.0.0 || ^10.0.0
+      typescript: '>=4.8.4 <6.1.0'
+
   typescript@5.9.3:
     resolution: {integrity: sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==}
     engines: {node: '>=14.17'}
@@ -1835,6 +1848,17 @@ snapshots:
     dependencies:
       prelude-ls: 1.2.1
 
+  typescript-eslint@8.58.2(eslint@9.39.4)(typescript@5.9.3):
+    dependencies:
+      '@typescript-eslint/eslint-plugin': 8.58.2(@typescript-eslint/parser@8.58.2(eslint@9.39.4)(typescript@5.9.3))(eslint@9.39.4)(typescript@5.9.3)
+      '@typescript-eslint/parser': 8.58.2(eslint@9.39.4)(typescript@5.9.3)
+      '@typescript-eslint/typescript-estree': 8.58.2(typescript@5.9.3)
+      '@typescript-eslint/utils': 8.58.2(eslint@9.39.4)(typescript@5.9.3)
+      eslint: 9.39.4
+      typescript: 5.9.3
+    transitivePeerDependencies:
+      - supports-color
+
   typescript@5.9.3: {}
 
   undici-types@7.16.0: {}