Add reusable Lighthouse CI workflow

[apermo]

Context

Themes and client-facing repos need Lighthouse CI checks on PRs to gate a11y scores and performance budgets.
This should be a reusable workflow so every consuming repo gets a consistent setup.

See apermo/idea-pool#31 for the broader a11y testing initiative.

Proposal

Create a new reusable-lhci.yml workflow with inputs:

• url or urls — the page(s) to audit (e.g. the DDEV site URL)
• a11y-threshold — minimum a11y score (default: 90)
• performance-threshold — minimum performance score (optional)
• config-path — optional path to a .lighthouserc.js in the consuming repo

The workflow should:

1. Set up the environment (Node, Chrome)
2. Run @lhci/cli against the target URL(s)
3. Report results as a PR status check
4. Upload the HTML report as a workflow artifact

Open questions

• Does the consuming repo need a running DDEV site, or can LHCI start its own server?
  For themes, the e2e workflow already boots DDEV — could LHCI piggyback on that, or should it be independent?
• Artifact retention policy for Lighthouse reports

[apermo]

Shipped in v0.4.0 via PR #20.