Add 'Investigating PRs and CI failures' guidance to copilot-instructions

[dsplaisted]

What

Adds a short Investigating PRs and CI failures section to .github/copilot-instructions.md:

• When asked to look at, investigate, or diagnose a pull request or build, first read the PR itself — its state, checks, and the conversation/comments — before deep analysis. Maintainers often post the root cause or the decided fix in the discussion, and a linked PR or issue may already carry it.
• If a PR's CI checks are failing, prefer the ci-analysis skill (.claude/skills/ci-analysis) for the deep dive — Azure DevOps/Helix failures, known-issue matching, and retry guidance — rather than ad-hoc investigation.

Why

When asked to "look at" a PR, the agent tends to jump straight into ad-hoc investigation and skips the PR discussion — where the answer often already is. Concrete example: diagnosing #54484 (a release/10.0.1xx → 10.0.3xx auto-merge failing Restore with '[]' is not a valid version string), the discussion already had the maintainer's root cause and the decided resolution (wait for the 10.0.9 servicing build to flow through the VMR, then pin the reference), and pointed at the linked PR that added the component. Reading the PR + conversation first would have landed on the right answer immediately.

The repo already ships a ci-analysis skill built for exactly this, but a generic prompt like "Look at the issue with #54484" doesn't match its CI-failure trigger phrases, so it isn't auto-selected. This guidance routes failing-CI investigations to it.

Why this shape

• Repo-scoped and conditioned on failing checks, so it won't over-trigger ci-analysis on code-review/explain requests.
• The first bullet is tool-agnostic — "read the PR + conversation first" helps regardless of which skill is right.
• Matches the file's existing "Title: + bullets" style; no behavior/code changes.

[Copilot]

Pull request overview

This PR updates the repository’s .github/copilot-instructions.md to add guidance for how agents should approach PR/build investigations, emphasizing reading the PR discussion first and (when applicable) routing CI-failure deep dives to a specialized skill rather than ad-hoc analysis.

Changes:

• Add a new “Investigating PRs and CI failures” section with two bullets: read PR state/checks/discussion first, and prefer a CI-analysis skill for failing checks.
• Keep the instructions file’s existing “Section title + bullets” structure and placement (inserted between Testing and Output Considerations).