ci(ofelia): clean obsolete inline workflows + preserve integration tests

Post-template-sync cleanup:

- Delete `auto-merge.yml` — superseded by `auto-merge-deps.yml` from
  the template (caller of netresearch/.github's org-level reusable).
- Delete `cleanup-containers.yml` — superseded by `container-retention.yml`
  (caller of ghcr-retention.yml reusable).
- Add `integration.yml` — not part of the go-app template because
  integration tests are ofelia-specific (Docker build + buildx
  requirement + integration build tag). Drift-check permits extra
  consumer files not present in the template, so no intentional-drift
  entry is needed.

Known follow-ups (not in this PR):
- Coverage threshold jumps 60% → 80% via template. Tests need to be
  expanded OR ci.yml added to intentional-drift with a lower threshold.
- workflow-lint job lost in the ci.yml overwrite; add back via a tiny
  lint-workflows.yml caller if desired.

Signed-off-by: Sebastian Mendel <sebastian.mendel@netresearch.de>

Author: CybotTM