Fix custom agent instructions

[shirasassoon]

This pull request makes significant improvements to the onboarding documentation for new item types in Fabric CI/CD. The changes provide clearer, step-by-step guidance for verifying API support, determining dependency order, handling inter-type references, and validating deployments. The documentation is now more actionable and reduces ambiguity for contributors.

Key improvements include:

Enhanced Eligibility Gate Verification

• Expanded the eligibility gates section to include explicit, step-by-step instructions for verifying Fabric API support and service principal authentication, including URL construction patterns and verification steps. This ensures contributors do not guess and always check the latest documentation.

Improved Dependency Handling Guidance

• Added detailed instructions for determining the correct position in the SERIAL_ITEM_PUBLISH_ORDER, including how to identify upstream and downstream dependencies, and provided an inter-type dependency map for reference.
• Introduced a new section explaining inter-type dependency handling, with patterns for resolving references to other item types (e.g., property lookup, path-to-ID, ID-to-name, and parameterization), and guidance on updating PROPERTY_PATH_ATTR_MAPPING when needed.

Deployment Validation and Sample Files

• Updated the guidance for sample files, emphasizing that only real exported items should be used, and outlined steps for validating deployments and updating integration tests with real data.

These updates make the onboarding process more robust and reduce the risk of errors when adding support for new item types.