fix(#2138): surface track_shipping push failures (review)

Review (MEDIUM): `git push ... 2>&1` did not check exit code, so a silent push
failure (auth-token expiry, network blip, non-fast-forward) would proceed to the
report step and declare success — silently reproducing the exact #2138 defect.
Add a fallback warning naming the rerun command so a failed push is visible
(best-effort: the PR already exists, so we still report it rather than abort).
Recaptures goldens + size baseline.

gsd-core/workflows/ship.md

@@ -402,7 +402,7 @@ would otherwise trigger (GitHub honors `[ci skip]` / `[skip ci]`):
 
 ```bash
 gsd_run query commit "docs(${padded_phase}): ship phase ${PHASE_NUMBER} — PR #${PR_NUMBER} [ci skip]" --files .planning/STATE.md
-git push origin ${CURRENT_BRANCH} 2>&1
+git push origin ${CURRENT_BRANCH} 2>&1 || echo "⚠ track_shipping: ship-note push failed — it is local-only; rerun: git push origin ${CURRENT_BRANCH}"
 ```
 </step>