Give future Backstage retirement journals an independent field owner #5043

🤖 Generated by the Agentic Engineer

Expected Behavior

New retirement journals have a dedicated field manager, independent of the application reconciler. The journal value, namespace identity, admission fence and retained data stay protected.

Actual Behavior

After the confirmed stateless retirement attempt in #4819, an application reconciliation tried to remove the journal and was correctly refused by its admission fence. The journal field was attributed to the application reconciler, although it is absent from that reconciler's desired namespace manifest. This blocks later deployments.

The DELETE probe payload defect is handled separately by #5040. Existing terminal journal maintenance belongs to #5042 and its active proposal #5044. This issue is narrowed to the future journal writer to avoid competing operations on the same live namespace. It grants no live mutation or retirement replay.

Steps to Replicate

1. Write a retirement journal without naming a dedicated field manager.
2. Observe that an existing application field owner can retain ownership of that field.
3. Reconcile the reviewed namespace declaration, which omits the private journal.
4. The namespace-journal fence correctly rejects removal.

Acceptance Criteria

• Name a dedicated field manager in the existing guarded journal writer.
• Exercise the real journal writer and the native kubectl request to prove that ownership choice reaches the API.
• Preserve the existing namespace UID/resource-version preconditions, journal payload and no-replay boundary.
• Leave existing terminal maintenance to Complete Backstage retirement by clearing the terminal journal #5042/fix(retirement): complete terminal Backstage cleanup #5044 and ownership/removal obligations under Retire the unused Backstage scaffold #4266.

Related to #4819 and #4266.