Mature skill evaluation infrastructure

[PureWeen]

Why

ASP.NET Core already has useful skill evaluations, but the repository contract around them is implicit: contributors have to infer which files are auto-discovered, where fixtures belong, how to run validation locally, and which failures represent skill quality versus model/tool infrastructure.

This PR turns that implicit arrangement into a small, repeatable repository convention. The goal is not to add another skill or expand evaluation policy. It establishes the shared foundation that current and future skill evals can use: a standard one-skill baseline-vs-skilled lane, an explicit boundary for specialized suites, one pinned local entry point, and deterministic checks that do not call a model.

Relationship to dotnet#68625

Draft PR dotnet#68625 publishes fix-challenge/try-fix reviewer skills and specialized reviewer evaluations. This PR deliberately does not cherry-pick, modify, or depend on that draft. It is based on the existing evals on current main and defines the general repository convention first.

If this structure proves useful, dotnet#68625 can conform to it later:

• one-skill generic A/B coverage lives at eng/skill-evals/<skill>/eval.vally.yaml and is auto-discovered;
• reviewer/fix-challenge suites with different orchestration or scoring remain specialized *.vally.yaml files invoked explicitly;
• fixtures stay beside the eval specs that consume them;
• eval-only runners and checks stay under eng/skill-evals, not in runtime skill directories;
• the same pinned Vally entry point and repository-layout checks apply without copying a reviewer-specific harness.

Keeping these changes separate lets the general infrastructure be evaluated on its own merits and avoids making the broader reviewer-skill draft the source of repository-wide conventions. This PR also does not claim to validate the quality of the reviewer skills in dotnet#68625; it only provides the topology and local tooling those evals can later use.

Summary

• document the general eng/skill-evals/<skill>/eval.vally.yaml convention and specialized-suite boundary
• add a cross-platform PowerShell entry point with exact @microsoft/vally-cli@0.13.0 identity
• add parser-free repository layout checks and deterministic failure-injection/runner self-tests
• validate experiment configuration with an isolated, model-free --compare --dry-run
• migrate removed per-stimulus pairwise graders to Vally 0.13 experiment-level comparison
• preserve each existing eval's effective 5 runs × 5 stimuli = 25 trials

Validation

• pwsh -NoLogo -NoProfile -File eng/skill-evals/run.ps1 Test
• pwsh -NoLogo -NoProfile -File eng/skill-evals/run.ps1 Validate
• PowerShell parser checks
• git diff --check

No hosted model or judge calls were made. Parser-dependent policy checks are explicitly documented as follow-ups rather than approximated with regex or prose heuristics.