Two reviewer.py reports flagged that workers had pr_diff.json /
pr_changes.diff but no `_pre_pr/` clone, forcing them to fall back to
diff-only review without a real before/after tree to simulate against.

Two root causes:

1. Partial-clone-of-partial-clone fragility. _clone_single_repo uses
   `git clone --filter=blob:none` (treeless), and the previous
   _setup_pre_pr_artifacts path then ran `git clone --no-hardlinks`
   from THAT treeless source followed by `git checkout --detach
   <merge_base_sha>`. On a partial clone the checkout step can't
   always materialize blobs at an arbitrary historic SHA, and we
   silently swallowed the failure.

2. Silent failure mode. The post-clone status block printed
   `<repo>_pre_pr/: present` only on success - the failure path
   printed nothing, so reports of "no pre_pr clone" couldn't be
   correlated with a specific reviewer.py log line.

Changes:

- Replace clone+checkout with `git archive --format=tar <sha> | tar
  -x -C pre_dir`. git archive only needs the trees + blobs for the
  one specified commit and will promisor-fetch them on demand from
  the partial-clone source. The result is a flat directory of files
  with no .git overhead, which is exactly what the worker needs
  (it diffs files - it doesn't need git history on the pre_pr tree).

- Add a fallback path: if the first archive attempt fails (genuinely
  missing blobs), explicitly `git fetch --filter=blob:none origin
  <merge_base_sha>` to populate the source's object store, then
  retry. GitHub supports `uploadpack.allowReachableSHA1InWant=true`
  so fetching by SHA works.

- Wrap both attempts in detailed error capture so any remaining
  failure surfaces with the actual git/tar stderr in the reviewer.py
  log instead of being silently dropped.

- Always print `<repo>_pre_pr/: present (N files)` OR `<repo>_pre_pr/:
  MISSING (workers will fall back to pr_diff.json...)` in the
  post-clone status block, parallel to the existing `.venv:
  present|MISSING` line. Now if the user sees "no _pre_pr clone" in
  a worker, they can scan the reviewer.py log for the matching line
  and the WARNING that explains why.

Smoke tested on unslothai/unsloth#4924 with a fresh treeless partial
clone: git archive extracts the merge-base tree (80 top-level files,
including install.sh) successfully.