chore(config): add specialist agent definitions for orchestrated delivery
Add five custom agent files to .github/agents/:
- orchestrator.agent.md — end-to-end delivery pipeline (classify, branch, plan, implement, commit, version bump, PR)
- planner.agent.md — read-only technical lead; produces ordered TDD implementation plans
- backend.agent.md — Kotlin/Spring Boot specialist (services, controllers, JPA, scheduler)
- frontend.agent.md — React/TypeScript specialist (components, pages, hooks, shadcn/ui)
- infra.agent.md — DevOps/architecture owner (Docker, Compose, Nginx, CI/CD, env vars)