feat: add optional best-practices and SEO score thresholds

[johnzook]

Summary

Adds two new optional inputs for asserting Lighthouse best-practices and SEO scores:

• lhci_min_score_best_practices — minimum best practices score (not asserted if omitted)
• lhci_min_score_seo — minimum SEO score (not asserted if omitted)

These follow the same pattern as the existing lhci_min_score_performance and lhci_min_score_accessibility inputs. They are fully optional with no defaults, so existing workflows are unaffected.

Why

Lighthouse audits already run best-practices and SEO checks, but the action only lets you set pass/fail thresholds for performance and accessibility. Theme developers who want CI gates for all four Lighthouse categories currently have no way to do so without a custom lighthouserc.yml.

Usage

- uses: shopify/lighthouse-ci-action@v1
  with:
    store: $\{{ secrets.SHOP_STORE }}
    client_id: $\{{ secrets.SHOP_CLIENT_ID }}
    client_secret: $\{{ secrets.SHOP_CLIENT_SECRET }}
    lhci_min_score_performance: 0.6
    lhci_min_score_accessibility: 0.9
    lhci_min_score_best_practices: 0.7
    lhci_min_score_seo: 0.9

Implementation

• New inputs are appended as assertions to lighthouserc.yml only when provided (keeps the heredoc clean and avoids empty assertions)
• Follows existing code patterns for input mapping and variable defaults
• Backwards compatible — no changes to default behavior

Testing

Validated in production CI on a Shopify store (zookanalytics/altareina) with all four categories configured and passing.