Fetch PR head ref to fix cherry-pick for fork PRs

The PR head SHA may not exist locally when the PR comes from a fork,
since git fetch origin only fetches base repo branches. Explicitly
fetch the PR ref via refs/pull/N/head so the SHA is available for
the cherry-pick revision range.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: holly-cummins

.github/workflows/syncbot.yml

@@ -31,8 +31,9 @@ jobs:
           git config user.name syncbot
           git config user.email github-actions@github.com
           
-          # Fetch all branches
+          # Fetch all branches and the PR head (which may be from a fork)
           git fetch origin
+          git fetch origin pull/${{ github.event.pull_request.number }}/head
           
           # Get the source branch name and target branch
           SOURCE_BRANCH="${{ github.head_ref }}"