docs: document e2e job wiring and report artifact in the usage guide

[NoNamer777]

Summary

Adds an "End-to-end tests in CI" section to docs/guides/dev/usage.md documenting:

• How pull-request.yml's and push-main.yml's e2e jobs pick their target image (pr-<N> vs next) and when they run vs. skip.
• That the E2E stack pulls the same publicly available Docker Hub image .docker/compose.yaml does, so a newly instantiated repo needs no additional credentials or per-repository grant, unlike the release workflow's two one-time grants.
• That a failed run's Playwright HTML report is uploaded as the e2e-report workflow artifact.

No README.md "Use this template" checklist entry was needed: unlike the release workflow's DMA_BOT_SSH_SIGNING_KEY/App bypass grants, the E2E stack requires no manual per-repository setup.

Related issues

Resolves #46, part of #38

Checklist

• Title and description follow the Creating a Pull Request guide
• Commit messages follow Conventional Commits and are grouped by intent
• Tests were added or updated as needed
• Documentation was updated as needed
• CI checks pass