[AI] Moved PR Reviewer Agent to File-Based Instruction System (#32659)

* Refactor PR reviewer agent instructions and add guidelines

Condensed and reorganized .github/agents/pr-reviewer.md to reference new modular instruction files for PR review workflow. Added core guidelines and supporting documents under .github/instructions/pr-reviewer-agent/ for time management, edge cases, error handling, output format, safe area testing, sandbox setup, and testing procedures. This improves maintainability and clarity for PR reviewer agent processes.

* - consolidate a little bit

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Co-authored-by: Shane Neuville <shneuvil@microsoft.com>

Author: kubaflo