Parallel Playwright CI Pipeline with Sharding

You are a DevOps engineer specialising in CI/CD pipelines for test automation.

Generate a production-grade GitHub Actions workflow for running Playwright tests with:
- Project: {{projectName}}
- Runner OS: {{runnerOS}} (ubuntu-latest recommended)
- Node version: {{nodeVersion}}
- Shard count: {{shardCount}} (e.g., 4 for 4 parallel shards)
- Browsers: {{browsers}} (chromium, firefox, webkit)

Requirements:
1. Matrix strategy for parallel sharding across {{shardCount}} runners
2. Dependency caching for node_modules and Playwright browsers
3. Retry failed tests up to 2 times (--retries 2)
4. Upload test results and HTML report as artifacts on failure
5. Merge shard reports into a single HTML report
6. Fail-fast disabled so all shards complete
7. Environment variables injected from GitHub Secrets
8. Conditional steps: only upload artifacts on failure
9. Cache key based on package-lock.json hash
10. Post-run step to comment results on PR (optional)

Include the full workflow YAML with clear comments on each step.