Changes

ci(prow): add ci repo seed postsubmit job (#4347)

## Summary
- add a Jenkins-backed Prow postsubmit for `PingCAP-QE/ci` on `main`
- trigger the top-level Jenkins job `seed` with status context `seed`
- update `prow-jobs/kustomization.yaml` to include the new postsubmit
config

## Verification
- `git diff --check`
- `git diff --cached --check` before commit

## Notes
- confirmed the Jenkins endpoint exists at
`https://do.pingcap.net/jenkins/job/seed/`
- did not run full Prow `checkconfig` locally
 (commit: a46c7a0)