Add AI Agent Guidelines for msphpsql

[David-Engel]

This pull request introduces a comprehensive new guidelines document for AI agents working with the msphpsql repository. The document, AGENTS.md, provides detailed architectural, build, testing, and contribution information to help both automated agents and developers understand the structure and workflows of the project.

Repository and Architecture Overview:

• Added AGENTS.md with clear explanations of the repository structure, including the shared C++ core and two PHP extensions (sqlsrv and pdo_sqlsrv).
• Documented key design patterns such as Zend API integration, platform abstraction, ODBC usage, and MARS support.

Build and Packaging Guidance:

• Included step-by-step build instructions for Linux/macOS (autoconf), Windows (PHP SDK + JScript), PECL packaging, and Docker environment setup.
• Explained version management and semantic versioning rules, including PECL-specific requirements.

Testing and CI/CD Practices:

• Provided detailed information on .phpt test formats, test organization, helper files, and categories covered by the test suite.
• Outlined CI/CD pipelines (Azure Pipelines, AppVeyor, Codecov) and internal CI/CD practices.

Contribution Workflow and Core Principles:

• Listed common tasks (bug fixes, adding options, version bumps, new