Render a report for GitHub Actions: workflow command annotations and a
Markdown job summary.

A report is a map of :workspace (a name), :bricks (measurements from the
metrics component), :violations (from the thresholds component),
:thresholds (the merged config), and optionally :graph-violations, the
violations the dependency graph draws when :violations leaves some out.
Violations are shown as findings (see thresholds/rows).