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/auth/src/di/bind-production.ts

@@ -6,6 +6,7 @@ import type { SanitizedConfig as _SanitizedConfig } from "payload";
 //
 // Until then it's a no-op that intentionally accepts (and ignores) the
 // SanitizedConfig argument so app-boot code can call it uniformly.
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
 export function bindProductionAuth(_config: _SanitizedConfig): void {
   // Default mock bindings from `module.ts` already loaded by container.ts;
   // nothing to swap.