fix(crow): make the trial-build gate no-op when the auto-patch branch is absent (#132)

## Problem

The trial pipeline errored:

```
git clone -q --branch auto/registry-patch-proposals ...
fatal: Remote branch auto/registry-patch-proposals not found in upstream origin
```

I deleted the stale `auto/registry-patch-proposals` branch during cleanup, and — now that cascade detection (#128) makes the proposer correctly find **no candidates** — the proposer exits before ever recreating it. So the branch is legitimately absent, and the gate's hard `git clone --branch` fails.

## Fix

"Branch absent" = "no pending auto-patch proposals" = **nothing to verify**, which should be a clean no-op, not a failure. Clone `main`, then fetch + checkout the patch branch only if it exists; otherwise log and `exit 0`. A present branch is verified exactly as before (checkout its tip, diff registry vs `main`).

This also means the gate now runs `main`'s `trial-build-registry.R` (with the #130 fix) rather than a possibly-stale copy on the branch.

Reviewed-on: #132

.crow/trial-build-registry.yaml

@@ -121,8 +121,13 @@ steps:
       # with BINCRAFT_VERBOSE_PATCH_BUILD support; harmless on older versions).
       BINCRAFT_VERBOSE_PATCH_BUILD: 'TRUE'
     commands:
-      - git clone -q --branch ${patch_branch} https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
+      # Clone main, then check out the auto-patch branch if it exists. When the
+      # proposer had no candidates it never (re)creates that branch, so a missing
+      # branch means "nothing to verify" -- no-op cleanly instead of failing the
+      # clone.
+      - git clone -q https://pat-s:$$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 ${patch_branch} branch; no pending auto-patch proposals to verify."; exit 0; fi'
       - mkdir -p /mnt/cache/R-pkgs
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
       - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R