docs(contributor-ladder): document Emeritus technical offboarding and reactivation (#2718)

## What this PR does

Expands the **Stepping Down / Emeritus Process** section in
`CONTRIBUTOR_LADDER.md`:

- Adds an explicit PR-based mechanism for moving a maintainer's row from
`Active` to `Emeritus` in `MAINTAINERS.md`.
- Adds a five-business-day technical offboarding checklist: GitHub team
membership, org role, direct collaborator entries, `CODEOWNERS`, Actions
secret rotation, internal channels.
- Defines a path for an Emeritus maintainer to return to active status.

This codifies the offboarding flow actually exercised in #2717 and
removes ambiguity for future maintainer lifecycle events. Useful
evidence for the CNCF Incubation Due Diligence review (Governance
section, maintainer-lifecycle criteria).

### Release note

​```release-note
NONE
​```

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated contributor documentation with details on the Emeritus
transition process, including offboarding procedures and reactivation
guidelines.

<!-- review_stack_entry_start -->

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/cozystack/cozystack/pull/2718?utm_source=github_walkthrough&utm_medium=github&utm_campaign=change_stack)

<!-- review_stack_entry_end -->

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

CONTRIBUTOR_LADDER.md

@@ -147,5 +147,22 @@ If and when contributors' commitment levels change, contributors can consider st
 
 Contact the Maintainers about changing to Emeritus status, or reducing your contributor level.
 
+### How the Emeritus transition is recorded
+
+1. A pull request is opened against `MAINTAINERS.md` that moves the maintainer's row from the `Active Maintainers` section to the `Emeritus Maintainers` section. The PR is opened by the maintainer stepping down or, if they have already become unresponsive, by another maintainer with their prior notification.
+2. The PR is reviewed under the standard rules in this document — for a voluntary transition, acknowledgement by at least one other maintainer is sufficient; for involuntary transitions, the majority vote described in [Involuntary Removal or Demotion](#involuntary-removal-or-demotion) applies.
+3. After the PR is merged, the project performs the following technical offboarding within five business days:
+   - Removes the user from any GitHub team that grants write or admin permissions on project repositories (currently the `engineering` team in the `cozystack` GitHub organization).
+   - Downgrades GitHub organization role from `admin` (Owner) to `member`, unless the user explicitly remains an organization admin in another capacity.
+   - Removes any direct repository-collaborator entries that granted elevated permissions beyond default organization membership.
+   - Updates `CODEOWNERS` if the user appeared there.
+   - Audits repository and organization Actions secrets; rotates any shared credentials that the user could have known (for example, registry tokens, CI bot keys, organization-level App private keys).
+   - Updates internal communication channels (private maintainer rooms, calendar invites) as appropriate. Emeritus maintainers remain welcome in public community channels.
+4. The Emeritus maintainer keeps default organization membership unless they ask to be removed. Their contribution history, commit attributions, and credits in past release notes are preserved unchanged.
+
+### Returning to active status
+
+An Emeritus maintainer who wishes to return to active maintainership opens a pull request moving their row back to the `Active Maintainers` section, with an updated affiliation if needed. The reactivation is treated like a new maintainer addition under [Becoming a Maintainer](#contributor-ladder) — the same review and approval flow applies. Past contributions count toward the new application; the contributor does not need to re-prove the full ladder.
+
 ## Contact
 * For inquiries, please reach out to: @kvaps, @tym83