Add fresh public agent-routing guidance

[Brandon Miller (brandom-msft)]

Summary

• single-commit replacement for blocked PR 929
• establish microsoft-foundry/foundry-samples as the sole venue for work intended for publication
• keep publication work on the normal public pull request, repository-check, and maintainer-merge route
• pause for repository-maintainer guidance when public-branch access is unavailable, without redirecting or recreating work in foundry-samples-pr
• prohibit retired destructive private paths and reserve the retained manual bridge and verified seed recovery for explicitly authorized operators

Scope

This PR changes only .github/copilot-instructions.md. It preserves the existing AI Platform Docs safeguards exactly and does not modify workflows, repository settings, CODEOWNERS, samples, notebooks, Azure DevOps work items, or any other pull request.

Tracking

• ADO 5502654
• ADO 5555657

Validation

• parsed .github/copilot-instructions.md with PowerShell ConvertFrom-Markdown
• git diff --check
• exactly one changed file: .github/copilot-instructions.md
• exactly one commit over current origin/main