strategy: drop residual "branch" terminology

Two small renames matching the refactor's intent that the topology is ref-based, not branch-based:

• common.go: "failed to ensure metadata branch" → "failed to ensure primary metadata ref".
• manual_commit_push.go: perf span "push_checkpoints_branch" → "push_checkpoint_refs", since PrePush iterates refs.Push (which may contain non-branch refs).