GM.HealthChecks

Standardized ASP.NET Core health checks for the GM.* ecosystem: a consistent liveness/readiness/startup tagging convention, JSON responses (status, per-check durations, tags), a generic external-HTTP dependency check, and one call — AddGMHealthChecks() + MapGMHealthChecks() — to wire /health/live, /health/ready and /health/startup. Add infra checks with the GM.HealthChecks.* provider packages.