Add Expert Angular Frontend Engineer agent

Add a new agent definition for an Expert Angular 21 Frontend Engineer (agents/expert-angular-frontend-engineer.agent.md) including metadata, expertise/guidelines, and extensive code examples demonstrating standalone components, Signals, zoneless change detection, inject(), reactive patterns, testing, routing, and performance best practices. Also update docs/README.agents.md to register the new agent with install badges and a short description.

Author: zshn-dev