Specify the step-runner

Summary

Adds comprehensive specification documentation for the step-runner system in docs/spec/. This establishes a spec-first development workflow by documenting the behavior, interfaces, constraints, and edge cases for all major components.

Why

This documentation enables spec-first development where behavior is defined before implementation, making it easier to understand, test, and maintain the step-runner codebase.

Review Process

This MR updates the GitLab Duo review instructions to make sure future code changes also update relevant specs.