Add reusable issue triage skill and strict Vally evals

[PureWeen]

Summary

This is layer 2 of native stack #68872 and depends on #68869.

• adds a narrowly triggered repository skill for classifying dotnet/aspnetcore issues
• moves the larger area-routing and ownership taxonomy into a progressively disclosed reference
• aligns current Blazor ownership by assigning src/Components/Forms/, src/Components/QuickGrid/, and src/Components/CustomElements/ to area-blazor; area-ui-rendering retains src/Razor/
• preserves the existing template/Identity disambiguation guidance
• defines the automated untyped contract as exactly Bug, Feature, or Task; the skill never recommends assigning Epic
• treats any trusted existing issue type as authoritative and reports it as preserved with no replacement recommendation
• retains the five existing frozen public Vally stimuli
• adds frozen coverage for a broad single Feature and an existing maintainer-assigned Epic
• hardens those two cases with independent named positive and negative output-matches graders; scoring: {} uses Vally's binary all-graders-pass verdict

Production wiring

Production is intentionally not wired in this layer. The workflow and generated lock are unchanged relative to #68869.

Validation

• Vally 0.13.0 strict lint passed for all repository skills and eval specs
• skill-eval layout validation and runner self-tests passed
• the skills-vs-baseline experiment dry-run discovered all 6 baseline/skilled plans
• all 7 issue-triage stimuli parse
• adversarial regex probes passed for good output, bare Type: Epic, preserve-Epic-plus-recommend-Feature, do not preserve Epic, and Type: Bug
• layer diff is restricted to .github/skills/issue-triage/** and eng/skill-evals/issue-triage/**
• no model-backed Vally trial or live issue mutation was run