Add deterministic replay gate for agent tests

[ZackMitchell910]

Summary

• add a replay-only RunLedger eval suite (suite/case/schema/cassette + stub agent)
• add a baseline file for regression gating
• add a GitHub Actions workflow using runledger/[email protected]
• add a small README note + ignore runledger_out/

How to run locally

runledger run evals/runledger --mode replay --baseline baselines/runledger-demo.json

Notes

• no external calls; replay-only cassette
• feel free to remove the suite/workflow if it is not desired

[ZackMitchell910]

Thanks for taking a look! This PR adds a replay-only RunLedger gate. The workflow runs are currently waiting on fork approval (action_required) or have not started yet for forks. If you are open to it, please approve/authorize the workflow run so CI can complete. Happy to adjust anything.