#1276 UPDATE.bat cannot update existing checkouts after the main history rewrite

描述

## Problem

The recent force-update of `main` prevents existing checkouts from updating through the normal `UPDATE.bat` workflow.

The rewritten history appears to remove Claude-related commit trailers. Regardless of the reason for that cleanup, existing users need a documented migration path.

**I do not normally use UPDATE.bat and did not run it to reproduce this failure.** I encountered the rewritten history while updating manually, then inspected UPDATE.bat to check the impact on users of the supplied updater. The behavior described below follows from the verified Git history and the script's control flow.

## Evidence

My checkout was on the previous `main` at:

`6f32ec070f23ced9f50e704d854d775da52591ab`

No local commits or rebases were made. After fetching, Git recorded:

```text
fetch --quiet origin: forced-update
```

The new upstream tip at the time of checking was:

`82f46a8c8f475f001ad76d92f58f4a4f8ffb0253`

`git merge-base` finds no common ancestor between these histories.

This is not limited to a few recent commits: the old history and the corresponding rewritten history each contain 845 commits, with no shared commit IDs. The root commit IDs also differ.

For example, old commit `6f32ec0` and its rewritten counterpart `a1641e9` have identical file trees, author dates and subjects. The latter omits the former's Claude-related trailers and has a different parent. This describes an observed difference, not a confirmed explanation from the maintainer.

## Effect on UPDATE.bat

`UPDATE.bat` runs `git pull --ff-only` and exits if that command fails.

Consequently, affected checkouts cannot fast-forward, and the script never reaches `setup.py --update`. The source, engine and Python packages remain at their previous versions.

The script's failure message says:

```text
git pull did not succeed (the reason is above): nothing was updated.
Files you changed here can stop it: "git status" lists them.
```

That message does not explain the upstream history rewrite or provide recovery instructions. Users may mistake the failure for a problem with their local changes, or simply stop trying and remain on an outdated version. Re-running UPDATE.bat does not resolve it.

The script does report a failure; it does not literally report "no updates available." The concern is that existing users lose the normal update path without actionable migration guidance.

## Requested improvement

- Announce the history rewrite and explain which existing checkouts are affected.
- Document a safe recovery procedure that preserves local changes, model configurations and downloaded model files.
- Have the updater detect this situation and clearly explain why the update failed and how to recover.
- Avoid automatically using `git reset --hard` or `git clean`, as users may have local modifications and custom configurations.

## Workaround used

I preserved the old branch and local changes, created a new branch tracking the rewritten `origin/main`, and then ran `setup.py --update`.

Related: #475, which introduced UPDATE.bat.