Add performance-investigation skill for local regression investigation

[Copilot]

Note

This PR was AI/Copilot-generated.

Adds a new performance-investigation skill focused on local performance investigation (building CoreRun, running BenchmarkDotNet against private builds, git bisect). Keeps the existing performance-benchmark skill intact for @EgorBot ad hoc PR benchmarking.

Changes

• New skill: .github/skills/performance-investigation/

  • SKILL.md — Local investigation workflow: building CoreRun at specific commits, --coreRun benchmarking, good/bad commit comparison, git bisect methodology. Explicitly defers @EgorBot usage to performance-benchmark.
  • references/local-benchmarking.md — CoreRun location (artifacts/bin/coreclr/<OS>.<Arch>.<Config>/), --coreRun usage, ResultsComparer, incremental rebuilds with CoreLib/libs.pretest caveat
  • references/bisection-guide.md — Feasibility check, bisect range identification, git bisect workflow with BenchmarkDotNet

• Restored: .github/skills/performance-benchmark/SKILL.md — @EgorBot ad hoc PR benchmarking workflow, unchanged from prior state

• Updated cross-references in copilot-instructions.md, issue-triage/SKILL.md, api-proposal/SKILL.md, jit-regression-test/SKILL.md to list both skills with clear scope boundaries

• Evals — 6 test cases covering local investigation, bisection, and negative cases (EgorBot requests correctly routed to performance-benchmark)

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[EgorBo]

I'm confused - I didn't create or ask Copilot to file this PR - I have no idea what it is about 😮

[steveisok]

I'm confused - I didn't create or ask Copilot to file this PR - I have no idea what it is about 😮

EgorBot is running wild! (Just kidding - I have no idea why)