Trailers in a -F commit message duplicated once; not reproducible

### What happened

Committing through the Bash tool with `git commit -F msg.txt`, where `msg.txt`
already ended with a block of git trailers, produced a commit whose message
contained that entire trailer block **twice**.

The message file was read back immediately before the commit and contained
exactly one copy. The trailers in question were five lines of the form:

```
Agent-Harness:
Agent-Model:
Agent-Session:
Agent-Review: unreviewed
Co-authored-by:
```

Amending with a trailer-free body then produced a commit carrying exactly one
copy of the block — i.e. the amend also gained trailers that were not in the
file it was given.

### It does not reproduce

Four later commits in the same checkout, same flow (heredoc to a file, then
`git commit -F `), with and without trailers in the file, each produced
exactly what the file contained and nothing more. I could not make it happen
again.

I am filing it anyway because the failure is silent — the commit succeeds, and
nothing in the output indicates the message differs from the file that was
supplied.

### What I ruled out

Nothing in the repository or git configuration explains an append:

```
git config core.hooksPath # empty
git config --global core.hooksPath # empty
git config --get-regexp 'trailer|commit\.' # empty
ls .git/hooks # only *.sample files
```

### Environment

* Claude Code 2.1.272
* macOS, darwin arm64
* git invoked through the Bash tool, non-interactive, message supplied via `-F`

### Why it matters

An agent instructed to write specific trailers has no way to tell that its
message was altered after the fact, short of reading the commit back with
`git cat-file` on every commit. If a harness-side append exists, making it
conditional on the trailer not already being present would make the two paths
agree.

---

🤖 Posted by an AI agent · harness: Claude Code 2.1.272 · model: claude-opus-5[1m]
Review: unreviewed
Session: 8d06d917-3d8f-425a-9a24-a480f24768e2

Start by reproducing the reported non-interactive Bash flow with git commit -F, both with and without trailers, then compare the supplied file with the commit message read using git cat-file. Done means the resulting commit matches the file exactly and does not duplicate or add trailers.

Written by the indexing model from the issue text.