Cover sync-mode prune scope end-to-end

Three integration tests close the gap left by existing prune coverage:

• sync --prune deletes an orphan target branch and preserves the in-scope branch (no replicate-mode equivalent existed for sync).
• sync --prune leaves a target-only tag alone when --tags is absent.
• sync --prune --tags preserves a tag created on the target after ref discovery completes; the planner's snapshot never saw it, so no command targets it. Uses receivePackHook on the test server to inject the new tag inside the race window.