GitHub Actions — Monorepo CI/CD Matrix

## Task
CI/CD pipeline for a monorepo with path-based triggers and parallel builds.

## Requirements
- Monorepo structure: packages/api, packages/web, packages/shared
- Path-based triggers: only build what changed
- Shared dependency caching
- Matrix builds for multiple Node versions
- Conditional deployment per package

## Workflow
```yaml
# Generate .github/workflows/ci.yml with:

on:
  push:
    branches: [main]
  pull_request:

jobs:
  detect-changes:
    # Use dorny/paths-filter to detect which packages changed
    # Output: api_changed, web_changed, shared_changed

  test-api:
    needs: detect-changes
    if: needs.detect-changes.outputs.api_changed == 'true'
    strategy:
      matrix:
        node: [20, 22]
    steps:
      # Checkout, setup Node, restore cache, install, lint, test, build

  test-web:
    needs: detect-changes
    if: needs.detect-changes.outputs.web_changed == 'true'
    steps:
      # Same pattern, plus Playwright E2E tests

  deploy-api:
    needs: [test-api]
    if: github.ref == 'refs/heads/main'
    # Deploy to staging → run smoke tests → promote to production

  deploy-web:
    needs: [test-web]
    if: github.ref == 'refs/heads/main'
    # Build → deploy to Vercel/Cloudflare → invalidate CDN cache
```

## Implementation Notes
1. Cache: pnpm store + turbo cache (shared across jobs)
2. Concurrency: cancel in-progress runs on same PR
3. Artifacts: upload test reports and coverage
4. Secrets: use environment-scoped secrets (staging vs production)
5. Notifications: Slack on failure, GitHub deployment status
6. Reusable workflows: extract common steps into .github/workflows/shared-*.yml