Add black-box container and deployment smoke tests

[darthfork]

Parent issue: #52

Summary

Add system-level smoke tests that validate promgithub as a built artifact/container instead of only as in-process Go code.

Scope

• Run promgithub as a container or built binary in an isolated test environment.
• Validate startup, config wiring, /health, /metrics, and basic webhook ingestion.
• Add deployment-oriented smoke coverage for Helm/chart install paths.

Phases

Phase 1: Container/binary smoke tests

• Start promgithub with required env vars.
• Verify health endpoint and metrics endpoint.
• Send a signed webhook and verify metric exposure.
• Verify startup failure when required config is missing.

Phase 2: Helm/deployment smoke tests

• Add helm lint / render validation.
• Install the chart into a disposable cluster (kind/k3d or similar).
• Verify pod readiness and endpoint availability.
• Cover both standalone and Redis-enabled chart modes where practical.

Design notes

• Keep this suite separate from fast PR tests if runtime is significant.
• Treat this as operator-perspective validation.
• Prefer a small number of high-value smoke paths over a huge matrix.

Acceptance criteria

• At least one black-box smoke path validates real startup and webhook ingestion.
• Helm/chart deployment gets automated smoke coverage.
• The suite is wired into CI at an appropriate cadence.