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/core-api/src/root.ts

@@ -1,7 +1,3 @@
-import type { AuthRouter } from "@repo/auth";
-import type { BlogRouter } from "@repo/blog";
-import type { MarketingPagesRouter } from "@repo/marketing-pages";
-import type { NavigationRouter } from "@repo/navigation";
 import { router } from "@repo/core-shared/trpc/init";
 import { authRouter } from "@repo/auth/api";
 import { blogRouter } from "@repo/blog/api";