Migrate repository agent rules from .clinerules to agents.md

[Copilot]

This PR migrates the repository's agent automation rules from the legacy .clinerules directory to a modern agents.md file structure, following GitHub's recommended practices for agent instructions and workflow automation.

Changes Made

Created agents.md

• Comprehensive guidelines: Consolidated all existing rules from .clinerules into a single, well-organized document
• Modern structure: Used markdown with YAML/JSON code blocks for configuration data as recommended by GitHub
• Improved organization: Added table of contents and logical sections for better navigation
• Enhanced documentation: Added context and examples to help contributors understand agent automation

Updated References

• Copilot instructions: Modified .github/copilot-instructions.md to reference the new agents.md instead of .clinerules
• Verified completeness: Ensured no other references to .clinerules exist in the repository

Removed Legacy Structure

• Clean migration: Deleted the entire .clinerules directory after successful content migration
• No data loss: All important rules and configuration have been preserved in the new format

Content Preserved

The migration maintains all existing guidelines:

• AI Assistant Workflow: Planning and implementation phases, tool usage guidelines, quality standards
• C# Development Standards: Formatting rules, nullable reference types, testing frameworks (xUnit SDK v2, Moq 4.14.x)
• IdentityModel Guidelines: Repository structure, shipped packages, API management with PublicAPI.Unshipped.txt requirements

Benefits

• Modern format: Follows GitHub's current best practices for agent configuration
• Better discoverability: Single source of truth at repository root level
• Improved contributor experience: Clear documentation with examples and structured configuration
• Enhanced automation: YAML blocks enable better parsing by automation tools
• Future-ready: Extensible structure for additional agent types and workflows

The new agents.md file provides the same comprehensive guidance while being more accessible and maintainable for both human contributors and AI agents.

Fixes #3312.

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