idempotent-redundancy

# Idempotent Redundancy When adding redundant paths (fallbacks, belt-and-suspenders), make them idempotent. ## Pattern Redundancy without idempotency causes loops, churn, or data corruption. ## DO - Use `_is_merge: true` for Braintrust updates - Check if value exists before writing (fallback only if missing) - Use atomic write/rename for file operations - Make reconciliation steps safe to run repeatedly ## DON'T - Write unconditionally in fallback paths - Allow multiple writers to overwrite each other - Fire "repair" actions that can trigger more repairs ## Source Sessions - a541f08a: "Redundancy is good only if idempotent" - 1c21e6c8: "Belt-and-suspenders, but make it idempotent" - 6a9f2d7a: "Idempotent repair hooks"