Add black-box container and deployment smoke tests

[darthfork]

Summary

• add black-box container smoke coverage for startup, health, metrics, signed webhook ingestion, and missing configuration
• add Helm/kind deployment smoke coverage for standalone and external Redis-backed modes
• wire system smoke tests into the deep CI lane and document local execution
• update Helm health probes and lock the supported Redis chart dependency

Verification

• SMOKE_IMAGE=ghcr.io/darthfork/promgithub:0.0.7 ./test/system/container-smoke.sh
• SMOKE_IMAGE=ghcr.io/darthfork/promgithub:0.0.7 SMOKE_KIND_CLUSTER=promgithub-smoke-fx-56c SMOKE_NAMESPACE=promgithub-smoke-fx-56c SMOKE_RELEASE=promgithub-smoke-fx-56c SMOKE_LOCAL_PORT=18058 ./test/system/helm-smoke.sh
• bash -n test/system/*.sh
• shellcheck test/system/*.sh
• git diff --check
• workflow YAML parsed with Ruby/Psych

Closes #56