chore: CHANGELOG fragment for GST-935

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

changelog.d/gst-935-ubi-pack-subdir.md

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+**`gstack-ubi sync` and `pack` work on a subdirectory of a git checkout (GST-935).** Packing any folder that sat inside a checkout but was not its root died with "does not appear to be a git repository", after the VM was already up, because pack fetched the checkout's refs from the subdirectory and git only fetches from a repository's root. The gstack-evals launcher syncs `quick/laneb/runner`, so every launch from a git checkout failed at its first sync.
+
+### Fixed
+- **A subdirectory packs its own files.** Pack ships the files git lists under that folder (tracked, edited and untracked, not ignored) with no `.git`. A whole-repository `.git` would show the VM every file outside the folder as deleted. Packing a checkout's root and packing a non-git folder work as before.
+- **A subdirectory git lists no files under is refused.** When the checkout ignores the folder, pack exits with "pack: git lists no files under <dir>" instead of uploading an empty folder, which a run could otherwise pass on.
+
+### Tests
+- `test/ubicloud-runner.test.ts`: packing a subdirectory of a checkout and of a linked worktree ships its tracked, edited and untracked files without a `.git` and leaves out ignored ones, and an ignored subdirectory is refused. This test fails on 1.92.58.0.