ci: add required-check migration helper

[rwgk]

Description

Related to #2652. This draft contains an optional required-check migration
helper and its mocked tests. The standalone pre-commit workflow was introduced
in merged #2994; #2993 implements the independent documentation link checker.

Add a read-only-by-default CLI to preview replacing the pre-commit.ci - pr
required check with Pre-commit (Linux), Pre-commit (Windows), and
Documentation links from GitHub Actions. Preserve unrelated rules, required
checks, enforcement, branch conditions, and bypass actors. Reject inherited
organization rulesets and unexpected App integration IDs.

Writing requires --apply, workflows merged into main, and successful
representative checks from the expected App and owning workflows. Re-read
affected rulesets immediately before writing and reject concurrent changes.
Merging this PR does not run the helper or change required checks or App access.

The migration guide has been removed from the stack and preserved
at its historical revision.
The current administrative plan is a manual cutover. This helper edits existing
rulesets; it does not create a main-only replacement ruleset. The manual
cutover does not require this helper.

Validation

The helper and its mocked tests are unchanged by the documentation cleanup.
Original local validation through Pixi on PATH with Python 3.14 passed all 123
standalone CI-tool tests and 36 subtests, including the helper and link-input
tests. The mocked tests do not change GitHub settings.

Earlier hosted validation on 7e1a772a413f34e7031ca5382f6bd80940cdd39b passed
the pre-commit workflow
on Linux and Windows and the
documentation workflow
for authored links, rendered HTML, and the aggregate gate.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[rwgk]

The helper here is one-time use only. Closing this PR, which will still keep the code around for reference by an admin, as an alternative to clicking buttons in the github web UI.