Add README and GitHub Actions build workflow

[Copilot]

Summary

Addresses the two missing pieces called out in the problem statement:

(a) Clear README

Adds a top-level README.md that covers:

• What the extension does and its five auditing panels
• Prerequisites (Node.js 20, pnpm 9, Chrome)
• Step-by-step installation and build instructions
• How to load the unpacked extension in Chrome DevTools
• Development workflow (pnpm dev watch mode)
• Quick-reference table of all available npm/pnpm scripts
• Notes on the OpenAI-powered color/chart audit panels
• Pointer to the evaluation/ sub-directory README

(b) Pre-built artifact via GitHub Actions

Adds .github/workflows/build.yml with a Build Extension workflow that:

• Triggers on every push and pull request to any branch, plus workflow_dispatch for manual runs
• Installs pnpm and Node.js 20 with dependency caching
• Runs pnpm install --frozen-lockfile and pnpm build
• Uploads the resulting dist/ folder as a downloadable artifact named site-accessibility-auditor (retained for 90 days)

This gives anyone a one-click way to download a pre-built copy of the extension directly from the Actions run without needing to clone the repository and build locally.

[gvegayon]

@copilot, fix the errors:

• https://github.com/EpiForeSITE/site-accessibility-auditor/actions/runs/25748133298/job/75623519883?pr=1
• https://github.com/EpiForeSITE/site-accessibility-auditor/actions/runs/25748109567/job/75623519807?pr=1

[Copilot]

@copilot, fix the errors:

• https://github.com/EpiForeSITE/site-accessibility-auditor/actions/runs/25748133298/job/75623519883?pr=1
  ...

Fixed in 2cb848f. pnpm/action-setup@v4 requires an explicit version when packageManager isn't set in package.json. Added version: 9 to the setup step (matching the lockfile format 9.0 already in the repo).

[gvegayon]

@copilot, try again, the latest run failed.

[Copilot]

@copilot, try again, the latest run failed.

Fixed in 628bd46. The new failure was ERROR packages field missing or empty — actions/setup-node@v4 runs pnpm store path to set up dependency caching, but pnpm 9 requires a packages field in pnpm-workspace.yaml for that lookup (the project's pnpm-workspace.yaml only has onlyBuiltDependencies). Removed cache: "pnpm" from the setup-node step to avoid the failing store path resolution.