feat: implement temporary code freeze with core-platform as sole CODEOWNERS (#6947)

## Explanation

During the messenger migration period, all changes require approval from
the core-platform team. This is a temporary measure to maintain control
during critical migrations.

Changes:
- Added code freeze notice to CODEOWNERS
- Set @MetaMask/core-platform as owner of all files (*)
- Commented out all existing ownership rules for preservation
- Original rules can be easily restored after migrations are complete

## References

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* Fixes #12345
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## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [ ] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Temporarily assigns @MetaMask/core-platform as owner of all files and
disables Yarn codeowner validation during migration.
> 
> - **CODEOWNERS**:
> - Add temporary code-freeze notice and set global `*
@MetaMask/core-platform`.
> - Comment out all previous per-package ownership rules for later
restoration.
> - **Tooling (Yarn constraints)**:
> - Disable codeowner validation by commenting out `expectCodeowner`
invocation and implementation in `yarn.config.cjs`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
febd307. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: cryptodev-2s