Create examples/_infra shared base compose

[patterncatalyst]

Summary

Create the shared infrastructure base that individual examples can extend or reference.

Tasks

• Create examples/_infra/ directory
• Create base compose files (LGTM, Postgres, Kafka, full)
• Create otelcol/config.yaml (OTel Collector config)
• Create grafana/datasources.yaml (Grafana datasource provisioning)
• Create db/init/ directory for shared Postgres init scripts
• Document shared infra in examples/_infra/README.md

Design

Individual examples use extends or include to pull from _infra/, then add their own services. Source templates from lgtm-podman-stack skill.