v1.91.126.0 fix(brain-sync): hold case-colliding artifact paths instead of reporting ok (#3220) (#3254)

* fix(brain-sync): verify requested index entries after add

Refuse missing or unrelated staged paths and stale indexed content without consuming the queue or resetting the index.

* fix(brain-sync): refuse new casefold path collisions

Check component prefixes in requested/index and outgoing trees, including retained commits and fetch/merge retries. Preserve historical aliases, local commits, index, and queue on refusal.

* fix(brain-sync): hold case-colliding paths instead of halting the sync (#3220)

Builds on Kkkkkuro's staged-index and casefold checks (#3245, from #3223).
A queued path whose spelling differs only by letter case from another path
in the index or the batch is held before staging: it stays queued, every
other file keeps syncing, and the status is "collision" rather than "ok".
An established spelling keeps syncing; only the new alias waits.

Skill start prints one ARTIFACTS_SYNC attention line naming the colliding
names and the repair anchor (docs/troubleshooting.md#artifact-case-collision);
gstack-doctor warns the same way. The outgoing check is cached by HEAD so
the empty-queue drain at every skill boundary stays near main's cost.
gstack-slug and skill start's slug path are unchanged.

Co-authored-by: Kkkkkuro <180382840+Kkkkkuro@users.noreply.github.com>
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(brain-sync): close review gaps in the case-collision hold

- Windows: write the held-path list with LF and strip CR from helper output,
  so CRLF cannot let a held path through or blank case_names.
- --publish-removals runs the outgoing case check before it pushes.
- The cached outgoing verdict is keyed by every ref, not HEAD alone, so a
  moved remote baseline is re-checked.
- The repair snippet folds only JSONL logs and lists snapshots and other
  files for a manual decision.

Co-authored-by: Kkkkkuro <180382840+Kkkkkuro@users.noreply.github.com>
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(brain-sync): second review pass on the case-collision hold

- The repair loop also folds the dropped spelling's untracked (held) files
  and says to check for leftovers before removing the folder on Linux.
- The cached outgoing verdict also keys on HEAD's symbolic target.
- The outgoing check no longer needs a temp file, so a failed mktemp can
  no longer skip it.

Co-authored-by: Kkkkkuro <180382840+Kkkkkuro@users.noreply.github.com>
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* test(brain-sync): give the retry-merge fixture a git identity

The retry merge commits with the ambient identity. CI and Ubicloud runners
have none under the test HOME, so the merge failed before the outgoing case
check ran and the drain reported push_failed instead of error.

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

* docs(changelog): unreleased entry for the #3220 case-collision hold

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

* v1.91.126.0 stamp

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

---------

Co-authored-by: KuroChen <Kkkkkuro@users.noreply.github.com>
Co-authored-by: Kkkkkuro <180382840+Kkkkkuro@users.noreply.github.com>
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/gbrain-sync-errors.md

@@ -44,6 +44,7 @@ cause, the held files and the fix.
 | `error` | The drain stopped before committing and kept the queue | `status=error. The last sync stopped before committing; the queue is kept and retried. See the cause: <bin> --status` | The cause `--status` names: a git index lock, a commit hook, a `git add` failure |
 | `unknown` | The status file holds a status code outside this table | `status=unknown. The sync status file is unreadable. See it: <bin> --status` | Run `<bin> --once` to rewrite it |
 | stale-push | No push for 24 hours while the drain's `drainable` count is above zero | `stale-push. Files are ready to sync but nothing has been pushed for over 24 hours. See why: <bin> --status` | The waiting reason `--status` names (a coupled group waiting for a consistent generation, a `git add` failure) |
+| case-collision | `case_names` is set: artifact paths differ only by letter case, so the drain holds queued files under them | `case-collision held=<N> names=<names>. Artifact paths differ only by letter case, which a Mac or Windows disk stores as one file; sync holds <N> queued file(s) under them and syncs everything else. Repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision. See the paths: <bin> --status` | Keep one spelling: the manual repair in [troubleshooting](troubleshooting.md#artifact-case-collision) |
 | stale-drain | `last_drain_at` older than 24 hours while the queue holds records | `stale-drain. The sync has not run for over 24 hours while files wait in the queue. See why: <bin> --status` | If no gstack process is running, remove `~/.gstack/.brain-sync.lock.d`, then run `<bin> --once` |
 
 `<bin>` is the absolute path, for example