ci: target testing branch for auto-merge group PRs

auto-merge group (brew, common, extensions, etc.) should land on testing
first — no review gate — and flow to main via weekly promotion. This
unblocks auto-merge which was stuck because main requires 1 review.

manual-merge and core-junctions still target main (require human review).

Assisted-by: Claude Sonnet 4.6 via pi

Author: castrojo

.github/workflows/track-bst-sources.yml

@@ -239,14 +239,22 @@ jobs:
             echo "*Generated by [Track BuildStream Sources](${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }})*"
           } > /tmp/pr-body.md
 
+          # auto-merge PRs target testing (no review gate, auto-merges to main via promotion).
+          # manual-merge PRs target main (require human review).
+          BASE_BRANCH="main"
+          if [ "${{ matrix.group }}" = "auto-merge" ]; then
+            BASE_BRANCH="testing"
+          fi
+
           EXISTING_PR=$(gh pr list --head "$BRANCH" --json number --jq '.[0].number' 2>/dev/null || true)
           if [ -n "$EXISTING_PR" ]; then
             echo "Updating existing PR #$EXISTING_PR"
             gh pr edit "$EXISTING_PR" --title "$TITLE" --body-file /tmp/pr-body.md
+            gh pr edit "$EXISTING_PR" --base "$BASE_BRANCH" 2>/dev/null || true
             PR_URL=$(gh pr view "$EXISTING_PR" --json url --jq '.url')
           else
             PR_URL=$(gh pr create \
-              --base main \
+              --base "$BASE_BRANCH" \
               --head "$BRANCH" \
               --title "$TITLE" \
               --body-file /tmp/pr-body.md)