Add protobuf workflow eval docs

[rforgeon]

Summary

Adds a small harness-neutral eval set for the Buf protobuf plugin.

What changed

• Add eval cases for schema evolution, Protovalidate design, and Buf config review
• Add a rubric covering compatibility, Buf ecosystem fit, actionability, and privacy boundaries
• Document optional Telvine publishing guidance with metadata-only telemetry events

Validation

for f in evals/*/cases.jsonl; do while IFS= read -r line; do jq -e . >/dev/null <<<"$line" || exit 1; done < "$f"; done
rg -n "Evals and production telemetry|evals/" README.md
git diff --check

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