chore: rename classifier to Kestrel

[hasandemirkiran]

Summary

• rename the model display name from k-cl-1 to Kestrel
• rename the machine-readable result key to kestrel
• rename the public per-example prediction artifact to kestrel.predictions.jsonl

Verification

• Python unit tests pass
• the renamed prediction artifact reproduces the unchanged F1 0.9344 result
• no k-cl-1 references remain in the public repository