docs: document GitHub Actions integration patterns (annotations, job summaries)

[christso]

Objective

Document how to integrate AgentV eval results into GitHub Actions workflows — annotations, job summaries, and PR comments — using post-processing scripts that parse AgentV output artifacts.

Design Latitude

• Add an Astro docs page or section covering GitHub Actions integration
• Include example workflow snippets for:
  • Parse results.jsonl → ::error:: / ::warning:: workflow commands for native annotations
  • Parse results.json → $GITHUB_STEP_SUMMARY markdown job summary
  • Use JUnit XML with existing GitHub Actions test reporters
  • Use --threshold flag (once feat(cli): --threshold flag for suite-level quality gates #698 lands) for CI quality gates
• Optionally create a reusable skill for annotation generation
• Keep this outside of AgentV core — CI integration is a downstream concern

Acceptance Signals

• Astro docs page with example GitHub Actions workflow YAML
• At least one example showing ::error:: annotation generation from results
• At least one example showing job summary generation
• Examples are copy-pasteable into a real workflow

Non-Goals

• No changes to AgentV core output pipeline
• No built-in annotation writer
• Not a GitHub Actions reusable action/workflow (separate scope if needed later)

Context

Originally proposed as a built-in GitHub annotations output writer. Reconsidered — annotation emission is a presentation concern better handled in the CI workflow itself. This matches how microsoft/skills does it: their skill-evaluation.yml reads results.json inline and emits ::error:: commands via a Node.js script step.

See microsoft/skills harness research.