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.

tests/workflow-size-baseline.json

@@ -71,7 +71,7 @@
   "settings-advanced.md": 40019,
   "settings-integrations.md": 15892,
   "settings.md": 33467,
-  "ship.md": 25107,
+  "ship.md": 25222,
   "sketch-wrap-up.md": 14267,
   "sketch.md": 20004,
   "smart-entry.md": 11124,