Fix stage source guard: fetch origin/dev before ancestry check

The shallow clone (fetch-depth: 2) doesn't include origin/dev, so
git merge-base --is-ancestor was silently failing. Fetch dev with
depth=1 first so custom merge commit titles still pass the check.

.gitea/workflows/stage.yaml

@@ -52,6 +52,8 @@ jobs:
           set -euo pipefail
           COMMIT_MSG=$(git log -1 --pretty=format:"%s" 2>/dev/null || true)
           echo "Merge commit: $COMMIT_MSG"
+          # Fetch dev so ancestry check works in the shallow clone.
+          git fetch --depth=1 origin dev 2>/dev/null || true
           # Gitea merge commits: "Merge pull request '...' (#N) from dev into stage"
           # Direct branch merges:  "Merge branch 'dev' into stage"
           # tea pr merge with custom title: any subject line is possible, so