Add Agent Merge CI guardrails to Copilot instructions

[jkoritzinsky]

Summary

• add repo-specific Agent Merge guardrails to .github/copilot-instructions.md
• forbid /azp and PR reopen reruns for failed CI
• require Build Analysis + ci-analysis to classify non-known failures, fix PR-caused issues, and open/update a Known Build Error issue for unrelated failures

Why

This keeps Agent Merge aligned with the dotnet/runtime workflow and prevents CI rerun churn from masking the original validation signal.

Note

This PR description was generated with GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
15 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/runtime-infrastructure
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Adds repo-specific guardrails to the Copilot instructions to prevent Agent Merge from masking CI signal through reruns, and to standardize how CI failures are triaged in dotnet/runtime.

Changes:

• Add an “Agent Merge / CI check resolution” section to document forbidden CI rerun actions (no /azp, no close/reopen reruns unless explicitly requested).
• Require using Build Analysis plus the ci-analysis skill to classify non-known failures, fixing PR-caused issues and filing/updating Known Build Error issues for unrelated failures.