Add Copilot cloud-agent onboarding guide with repo-specific workflow and asset prerequisites

[Copilot]

This PR onboards the repository for first-time Copilot cloud agents by adding a dedicated .github/copilot-instructions.md with high-signal operational context. It also documents a real failure mode observed in fresh clones (missing standalone assets) and the practical workaround.

• What was added

  • New onboarding file: .github/copilot-instructions.md.
  • Repository-specific guidance for:
    • core architecture and ownership of key paths (src/Weasyprint.Wrapped, tests, examples),
    • canonical build/test entry points used by CI,
    • CI workflow shape (build-test-code-scan.yml) and artifact expectations,
    • packaging/release coupling to root asset folders.

• Known failure mode now documented

  • Captures the integration-test failure when assets/standalone-linux-64.zip is absent in fresh environments.
  • Records the corresponding workaround to generate required assets before running tests.

• Agent-focused operating guidance

  • Calls out high-impact files for behavior changes (Printer, ConfigurationProvider, integration tests).
  • Highlights invariants to preserve (cross-platform asset naming, OS-specific execution paths, stderr filtering behavior).

Example snippet now reflected in the onboarding guide:

# run from repository root
./build-on-linux.sh
dotnet test --verbosity normal ./src/Weasyprint.Wrapped.Tests/Weasyprint.Wrapped.Tests.csproj