feat(eslint-config): add boundaries plugin enforcing app→feature→core graph

- Install eslint-plugin-boundaries@^4.2.2 to enforce three-tag boundary model
- Configure element types: app, core-composition (core-api/core-cms), core, feature, tooling
- Enforce unidirectional dependency graph: apps→features→core, core-composition→features
- Add eslint.config.js to all 17 packages and apps (required for ESLint 9 flat config)
- Fix pre-existing linting issues to achieve clean lint pass

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/eslint-config/base.js

@@ -2,9 +2,10 @@ import js from "@eslint/js";
 import eslintConfigPrettier from "eslint-config-prettier";
 import tseslint from "typescript-eslint";
 import turboPlugin from "eslint-plugin-turbo";
+import boundaries from "eslint-plugin-boundaries";
 
 export default [
-  { ignores: ["dist/**", "node_modules/**"] },
+  { ignores: ["dist/**", "node_modules/**", ".next/**", ".turbo/**"] },
   js.configs.recommended,
   ...tseslint.configs.recommended,
   eslintConfigPrettier,
@@ -14,4 +15,33 @@ export default [
       "turbo/no-undeclared-env-vars": "warn",
     },
   },
+  {
+    plugins: { boundaries },
+    settings: {
+      "boundaries/elements": [
+        { type: "app", pattern: "apps/*" },
+        { type: "core-composition", pattern: "packages/core-api" },
+        { type: "core-composition", pattern: "packages/core-cms" },
+        { type: "core", pattern: "packages/core-*" },
+        { type: "tooling", pattern: "packages/eslint-config" },
+        { type: "tooling", pattern: "packages/typescript-config" },
+        { type: "feature", pattern: "packages/!(core-*|eslint-config|typescript-config)" },
+      ],
+    },
+    rules: {
+      "boundaries/element-types": [
+        2,
+        {
+          default: "disallow",
+          rules: [
+            { from: "app", allow: ["app", "core", "core-composition", "feature", "tooling"] },
+            { from: "feature", allow: ["core", "tooling"] },
+            { from: "core", allow: ["core", "tooling"] },
+            { from: "core-composition", allow: ["core", "feature", "tooling"] },
+            { from: "tooling", allow: ["tooling"] },
+          ],
+        },
+      ],
+    },
+  },
 ];