Add Helm chart for Kubernetes deployment #84

Summary

Ship a standalone Helm chart inside the repo so users can deploy agent-control to Kubernetes with helm install.

Currently the only supported deployment method is Docker Compose, which works for local dev but isn't production-grade. A Helm chart would give users health-based restarts, rolling deploys, horizontal scaling, and a standard configuration interface.

Motivation

• Agent-control is stateless with a single dependency (PostgreSQL) - ideal for a simple, maintainable chart
• Users deploying to production will almost certainly be on Kubernetes
• The server requires database migrations on startup (alembic upgrade head), which needs an init container to avoid race conditions on multi-replica deploys