ci: make main the canonical branch (badges guard, docs)

Point the badges job's branch guard at refs/heads/main and update the README /
ci-cd docs to name main as the canonical branch, ahead of establishing main from
bringup as the SDK's first main commit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf

.github/workflows/ci.yml

@@ -132,7 +132,7 @@ jobs:
 
   badges:
     needs: [test]
-    if: github.event_name == 'push' && github.ref == 'refs/heads/bringup'
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     permissions:
       contents: write