fix(crow): probe the auto-patch branch with git ls-remote to avoid a scary fatal (#133)

Follow-up to #132. The no-op path already worked (the `else` branch runs and `exit 0`s), but using `git fetch` as the *existence check* printed `fatal: couldn't find remote ref auto/registry-patch-proposals` to stderr — which looks like a failure even though the step succeeds.

Probe with `git ls-remote --exit-code --heads origin ${patch_branch}` instead: verified against the live repo it exits 0 for an existing branch and 2 for a missing one, **silently** (no `fatal`). Only fetch + checkout when the branch actually exists.

Net: same behavior, clean log — a run with no pending auto-patch branch prints just `No ... branch; no pending auto-patch proposals to verify.` and exits 0.
Reviewed-on: #133

.crow/trial-build-registry.yaml

@@ -127,7 +127,7 @@ steps:
       # clone.
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - git fetch -q origin main
-      - 'if git fetch -q origin ${patch_branch}; then git checkout -q FETCH_HEAD; else echo "No ${patch_branch} branch; no pending auto-patch proposals to verify."; exit 0; fi'
+      - 'if git ls-remote --exit-code --heads origin ${patch_branch} >/dev/null 2>&1; then git fetch -q origin ${patch_branch} && git checkout -q FETCH_HEAD; else echo "No ${patch_branch} branch; no pending auto-patch proposals to verify."; exit 0; fi'
       - mkdir -p /mnt/cache/R-pkgs
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
       - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R