CI: add e2e test workflow

[kraftbj]

Summary

• Adds an e2e test workflow to CI that runs on PRs and pushes to trunk
• Decouples e2e tests from the Jetpack monorepo: removes the unused _jetpack-e2e-commons workspace dependency and replaces the monorepo build script with a no-op
• Generates a pnpm-lock.yaml for reliable dependency caching in CI
• Tests run via Docker (WordPress + MariaDB) using Jest

Test plan

• Verify the e2e workflow runs successfully on this PR
• Confirm pnpm install --frozen-lockfile succeeds with the committed lockfile
• Confirm Docker environment starts and WordPress becomes available
• Confirm e2e tests execute and pass