ci: add shreds e2e cross-repo workflow

[packethog]

Summary

• Add a GitHub Actions workflow that runs doublezero-shreds e2e tests against the current doublezero commit on every PR and push to main
• Checks out the doublezero-shreds repo, overrides the pinned doublezero SHA in e2e/.dep-shas.json, builds shreds e2e container images, and runs the test suite
• Uses a separate GHCR namespace (dz-shreds-e2e) to avoid tag collisions with the shreds repo's own CI
• Pins the shreds checkout SHA between setup and e2e jobs to prevent race conditions

Testing

• Passes in CI