feat(yield): add evaluation evidence surface

[bigboateng]

Summary

• add six pinned skill-conversion cases with the thin skill and Yield program visible side by side
• add a reproducible source-size harness and small website-safe result summary
• validate conversion source token counts, summary arithmetic, source identities, and artifact publication status
• keep raw transcripts, worktrees, and large artifacts out of Git while requiring a URI and digest for promoted behavior results
• validate the same surface in the projected public Yield repository

Test plan

• npm ci && npm run validate in yield/evals
• npm run measure against all six pinned upstream source hashes
• go vet ./...
• go test ./...
• project the public Yield tree and rerun the eval validator from the projected output

Locus comparison: same-repo definitions plus external raw artifacts was the only nonblocking modeled candidate. Separate-repo drift and raw-in-Git growth produced explicit blocking states. The result is theorem-only and the implementation discharges the immediate storage boundary, not the future Harbor retention policy.