feat: add functional tests for GitHub workflow action blocks

[jeff-schnitter]

Summary

Add functional tests for all 41 GitHub workflow action blocks in Cortex. Each test creates a one-step Cortex workflow, triggers it via the Workflows Run API, and verifies both the Cortex run status and the actual GitHub side-effect.

Scope

• 41 workflow YAML definitions in data/functional/workflows/
• New CLI commands: cortex workflows run, cortex workflows get-run, cortex workflows list-runs
• Test infrastructure: fixtures, helpers, safeguards, sweep job
• Justfile recipes for running functional tests

Design Spec

See docs/superpowers/specs/2026-04-06-github-workflow-action-block-tests-design.md