Add push client that sends run summary to a configurable HTTP endpoint
on shutdown. Execution context is read from GitHub Actions style
environment variables (with Gitea fallbacks).
New flag: -push-endpoint <url>
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
slog.Any() does not properly serialize slices of slog.Group() attributes,
resulting in broken output like {"Key":"","Value":{}}. Fixed by passing
structs with JSON tags directly to slog.Any() instead.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
