feat: GitHub Action for CI/CD architectural health checks

[bntvllnt]

Summary

Publish a GitHub Action that runs codebase-intelligence as a CI check on PRs — "Architecture check passed/failed" as a PR status check.

Motivation

• dependency-cruiser's CI integration is a major adoption driver
• "Architecture check failed" in PR checks = automatic enforcement
• GitHub Actions is the simplest distribution channel for CI tools
• Repomix has GitHub Actions integration; we should too

Features

1. PR check: Run on pull_request events, report pass/fail
2. Metrics delta: Compare PR branch metrics vs base branch
3. Threshold enforcement: Fail if coupling > X, blast radius > Y
4. Comment summary: Post metrics summary as PR comment
5. Badge: Dynamic badge for README (architectural health score)

Example Workflow

- uses: bntvllnt/codebase-intelligence-action@v1
  with:
    path: ./src
    fail-on:
      coupling-increase: true
      new-dead-exports: true
      blast-radius-threshold: 50

Acceptance Criteria

• GitHub Action published to Marketplace
• Runs on PR events, posts comment with metrics
• Configurable thresholds for pass/fail
• Delta comparison (PR vs base branch)
• README badge support

Priority

Medium-term — High adoption potential but requires stable CLI first.