Managed ILM policies are hidden by default behind a Managed filter in retention policy selectors

## Summary

When selecting an ILM policy for data retention, both in Index Management (Data Streams, Edit data lifecycle) and in Streams (Data lifecycle tab), policies preconfigured and managed by Elastic are now hidden from the policy list by default. A new Managed filter toggle next to the search input reveals them, and revealed managed policies carry a Managed badge (also shown in the inspect-policy flyout title). A stream or data stream that already inherits a managed policy keeps that policy visible in the selector. Users following the documented select-a-policy workflow will not see managed policies (such as built-in logs or metrics policies) until they activate the toggle.

**Why this needs docs:** Why this needs docs: the published Streams retention workflow says to choose a pre-defined policy from the list, and managed policies no longer appear in that list by default, so the documented steps silently miss a whole class of policies.

## Resources

- PR [#278435](https://github.com/elastic/kibana/pull/278435) — [Streams][Index Management] Add managed badge to retention selector

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.5.1, v9.6.0 |
| **Serverless** | N/A |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @florent-leborgne*

## Suggested edits

[Configure data retention with Streams](https://www.elastic.co/docs/solutions/observability/streams/configure-retention) > Follow an ILM policy
- **What the docs say:** "Select ILM policy, then choose a pre-defined policy from the list."
- **What to add:** Add a sentence (tagged paragraph) explaining that policies managed by Elastic are hidden from the policy list by default and that users can select the Managed filter next to the search field to show them; managed policies display a Managed badge, and a stream that already follows a managed policy keeps it visible in the selector. The same selector behavior applies when editing a data stream's data lifecycle in Index Management. Stack only, from 9.6.0 (not available in serverless).

Open “Configure data retention with Streams” and locate the “Follow an ILM policy” section, then review the existing pre-defined policy instruction. Update it for Stack 9.6.0 to explain the Managed filter, Managed badge, and visibility of already-managed policies, and note that the same behavior applies in Index Management but not Serverless.

Written by the indexing model from the issue text.