Add automatic cleanup of old core files using WordPress $_old_files list

[Copilot]

Feature: Add support for deleting old core files using WordPress $_old_files list

Problem

Currently, when checksums for the target WordPress version and locale aren't available, WP-CLI warns users to manually clean up old files. This is inconvenient and error-prone.

Solution

This PR implements automatic cleanup using WordPress core's $_old_files global array from wp-admin/includes/update-core.php.

Cleanup Order

Old files from WordPress core's $_old_files list are always cleaned up first, before fetching and comparing checksums. If checksums are available, the checksum-based diff cleanup follows.

Changes Made

1. Modified cleanup_extra_files() - cleanup_old_files() now runs unconditionally at the start
2. cleanup_old_files() - runs unconditionally
3. Removed cleanup_old_files_not_in_checksums() - now redundant
4. Added get_old_files_list() - centralizes access to $_old_files array
5. Added remove_old_files_from_list() - single-pass helper with early exit and path validation
6. Added remove_directory() - uses RecursiveIteratorIterator/RecursiveDirectoryIterator with CHILD_FIRST ordering
7. Updated command docblock examples to reflect new cleanup output:
   • Standard update example: No files found that need cleaning up → No old files were removed.
   • Force to 3.1 example: added No old files were removed. and the warning about checksums being unavailable
8. Updated test for "Update from a ZIP file" to include new cleanup log messages

Testing

• Two comprehensive Behat test scenarios for new functionality
• "Update from a ZIP file" scenario updated for new log messages
• No breaking changes
• Backward compatible

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Way to delete old core files after core update</issue_title>
<issue_description>## Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing
Currently there is no way to delete old core files if during the update checksums for target WP version and locale aren't available.
Describe the solution you'd like
Command or a flag to delete old core files.
A clear and concise description of what you want to happen.
A way to delete old core files after core update exists.
</issue_description>

<agent_instructions>The list of old files in core can be retrieved by including wp-admin/includes /update-core.php and then accessing the global $_old_files variable</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@schlessera WP-CLI currently diffs the old checksums against the new checksums to decide what left-overs to purge in https://github.com/wp-cli/core-command/blob/1fa06e0cdb5d378585a69d01c94ad2d33278cf28/src/Core_Command.php#L1350-L1393

However, for combinations of locales & versions that we don't have checksums available, this just fails with a message that cleaning up by hand is required.

However, WordPress provides a list of files that are supposed to be deleted and deletes them by default on update: https://github.com/WordPress/wordpress-develop/blob/a216b6d83845dc4cdd01ff517cdf36b72043b6b3/src/wp-admin/includes/update-core.php#L20-L791

We should combine this data with the checksums, to use the checksums to only delete old files when they have not been modified, and to unconditionally delete old files when we don't have checksums to verify for modifications.</comment_new>
<comment_new>@schlessera
@Aaron-Ritter This is indeed still an issue. The latest update for this is that release 5.9 showed that the WordPress update process is broken as well and will be rethought for 6.0. Hopefully, that rethinking can also include WP-CLI-usable logic that just works.</comment_new>

• Fixes Way to delete old core files after core update #141

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[codecov]

Codecov Report

❌ Patch coverage is 58.94737% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/Core_Command.php	58.94%	39 Missing ⚠️

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[wojsmol]

we also need to check that this will not cause a regression of #196