saving body-only edit deletes index.json without re-adding it (entryLayout: 'content' + format: {data:'json'})

Description

When saving an entry that uses entryLayout: 'content' with format: { data: 'json' }
(metadata in index.json, body in a separate content.mdoc/markdown file), editing
only the body content sometimes causes Keystatic to emit a Git commit that
deletes index.json without re-adding it, even though the metadata itself was
unchanged. Content.mdoc updates correctly; index.json is just gone.

Environment

• @keystatic/core: 0.6.9
• @keystatic/astro: 6.0.0
• Storage: kind: 'github'
• Collection: path: 'src/content/blog/*/', entryLayout: 'content', format: { data: 'json' }

Reproduction

1. Open an existing entry in the GitHub-storage-mode admin UI.
2. Edit only the markdoc body field (leave title/metadata untouched).
3. Click Save.
4. Inspect the resulting commit: index.json is deleted, content.mdoc is
   modified in place — no addition recreates index.json.

Evidence

Commit example (repo private, redacted path):
content.mdoc | 2 ++
index.json | 24 ----------------------
2 files changed, 2 insertions(+), 24 deletions(-)
delete mode 100644 .../index.json

A follow-up save then fails with:
[GraphQL] A path was requested for deletion which does not exist as of commit oid ''
because index.json was already deleted by the prior save.

Suspected root cause

In useUpsertItem (keystatic-core-ui.js), filesToDelete is derived from
args.initialFiles (a snapshot captured when the entry form was loaded) and
additions is filtered by comparing blob sha against unscopedTree
(a client-side cached tree). If that cached tree is stale relative to the
actual GitHub HEAD (e.g. after a previous save, or a tab left open), an
unchanged file can end up filtered out of additions while still remaining
in filesToDelete, producing a commit with a deletion but no matching
re-addition.

Related

Same symptom as #1269, which was closed without a root cause. This report
includes a concrete reproducible commit diff.

Impact

Data loss — entries silently disappear from the CMS (Entry not found,
0 entries) until manually restored from git history.

Inspect useUpsertItem in keystatic-core-ui.js, focusing on how args.initialFiles, additions, filesToDelete, and the cached unscopedTree interact during a body-only save. Reproduce the GitHub-storage edit from the issue and verify that saving unchanged metadata retains index.json while updating content.mdoc, including on a follow-up save.