[engsys] Consolidate agent guidance into AGENTS resolver

[JennyPng]

Description

Consolidates repository-level agent guidance using the resolver structure adopted by azure-sdk-for-js#38927.

• Replaces the broad AGENTS.md handbook with a concise routing table and repository-wide hard rules.
• Keeps .github/copilot-instructions.md as a compatibility redirect.
• Moves management-plane review checks into a path-scoped reviewer instruction.
• Moves package-health reporting guidance into a dedicated prompt.
• Updates release-readiness and package-skill references to current skills and MCP tool names.
• Removes stale tool names, duplicated workflows, fixed time estimates, and incorrect docstring guidance.

The management reviewer instruction also makes the existing preview API/package-version requirement explicit in its scoped canonical location.

Engineering-system follow-up

eng/common/pipelines/templates/jobs/ai-eval-job.yml currently passes .github/copilot-instructions.md directly to the external MCP evaluation harness. The redirect follows the JavaScript repository precedent, but the eval owners should confirm whether that harness resolves AGENTS.md or should be updated in the shared engineering-system source.

Validation

• Ran eng/common/scripts/Verify-Links.ps1 against all changed Markdown files: 11 links checked, no broken links.
• Ran git diff --check.

All SDK Contribution checklist

• The pull request does not introduce breaking SDK changes.
• CHANGELOG update is not required because this only changes agent guidance.
• I have read the contribution guidelines.