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/fixtures/golden-install-parity/opencode.json

@@ -363,7 +363,7 @@
   "gsd-core/workflows/settings-advanced.md": "252b0d3edc315339",
   "gsd-core/workflows/settings-integrations.md": "d1711a95f44fdbde",
   "gsd-core/workflows/settings.md": "2e42ee34c791378a",
-  "gsd-core/workflows/ship.md": "61e695596f2dd605",
+  "gsd-core/workflows/ship.md": "3a80ea841f0a81b9",
   "gsd-core/workflows/sketch-wrap-up.md": "681800323681c5c6",
   "gsd-core/workflows/sketch.md": "fcb7af914159ef7b",
   "gsd-core/workflows/smart-entry.md": "2a253fe437496eea",