feat: add HF agent trace downloader and compaction verifier

- prepare_hf_agent_traces.py: streams from Toucan-1.5M, nebius SWE-agent,
  glaive function-calling, Claude code traces; serializes and windows
  into 8K token chunks for labeling pipeline
- verify_compaction.py: reconstructs before/after from labeled records,
  shows unified diff + stats, optional Cursor diff view