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/package.json

@@ -14,6 +14,7 @@
     "@typescript-eslint/parser": "^8.25.0",
     "eslint": "^9.20.0",
     "eslint-config-prettier": "^10.1.0",
+    "eslint-plugin-boundaries": "^4.2.2",
     "eslint-plugin-turbo": "^2.4.0",
     "typescript-eslint": "^8.25.0"
   }