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/blog/eslint.config.js

@@ -0,0 +1,3 @@
+import baseConfig from "@repo/eslint-config/base";
+
+export default baseConfig;