chore(eval): add migrate-to-rstest eval and report

[fi3ework]

Summary

• Add skills-test/migrate-to-rstest/evals/evals.json and report.md, preserving the migrate-to-rstest evaluation data that was previously kept on the migrate-to-rstest-eval branch before the evaluator infra (chore(infra): add rstack-skill-evaluator skill #49) landed.
• Drive-by: normalize rstack-skill-evaluator's digest in skills-lock.yaml from a sha256 to "", matching how skills-package-manager v0.9.0 (chore(deps): upgrade skills-package-manager to v0.9.0 #52) treats link: entries (same as pr-creator). This is what pnpm install writes locally — the previous value was a leftover from chore(infra): add rstack-skill-evaluator skill #49 being branched off pre-v0.9.0.

The eval evals.json schema extends the existing one (rsbuild / rslib) with fixture_root, runs_root, notes, plus per-eval fixture / pre_migration_baseline, because rstest fixtures live outside the repo (under /tmp/agent-skills-evals/...) rather than as in-tree projects. Core fields (skill_name, evals[], eval_name, prompt, assertions) match.

The skill code itself (skills/migrate-to-rstest/SKILL.md and references/) is intentionally not brought over — main already has the more refined version from #51 ("tighten principles and defer to official guides"), and the branch's older copy would regress it.

Test plan

• CI green
• Confirm evals.json schema is acceptable to the rstack-skill-evaluator skill (or the schema is documented as evolving)

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