Point staging back at main builds

[ecv]

Summary

Staging pulls its image from a filter pinned to the class branch, so anything merged to main is invisible there. The rollback filter is already in the file, commented out: https://github.com/datum-cloud/infra/blob/main/apps/dns-operator/downstream/staging/ocirepository-patch.yaml

This has to land after the merge, otherwise staging drops to the last main build and loses the class changes it is running today.

Acceptance criteria

• Staging resolves its image from main builds.
• Flux reconciles the source and the running image matches the merge commit.
• No manual pin is left behind for the next person to discover.

Related to #95