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/Runledger@v0.1
• 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
• GitHub Actions note: workflows from first-time contributors/forks may require a maintainer to click “Approve and run” before checks will execute.

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Latest commit: 03c5111

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