[WIP] Add copilot skill based on ci-analysis

[Copilot]

• Explore the dotnet/runtime ci-analysis skill (PR #123913) to understand structure and content
• Explore the dotnet/diagnostics repo structure, CI pipelines, and existing copilot setup
• Create .github/skills/ci-analysis/SKILL.md - adapted for diagnostics (no known failures/Build Analysis, default to dotnet/diagnostics)
• Create .github/skills/ci-analysis/scripts/Get-CIStatus.ps1 - PowerShell script adapted for diagnostics defaults
• Create .github/skills/ci-analysis/references/azdo-helix-reference.md - adapted with diagnostics pipeline info
• Create .github/skills/ci-analysis/references/helix-artifacts.md - Helix artifacts reference (largely same)
• Create .github/skills/ci-analysis/references/manual-investigation.md - manual investigation guide (largely same)
• Run code review
• Run CodeQL check

Original prompt

Lets add a copilot skill based on the ci-analysis skill in the dotnet/runtime repo. See the PR that added that skill: dotnet/runtime#123913

The relevant skill in the dotnet/diagnostics repo will be similar but not exactly the same as we don't have known failures set up in this repo.

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