docs: add architectural caveats for data sources

Jiaan Louwrequested to merge
jlouw-expand-agents-md-with-architecture into main

What does this MR do and why?

Expands AGENTS.md with additional steps and architectural caveats to consider when adding a new data source.

Includes missing common steps from the POC findings in #78.

Related issue: Improve developer experience for adding GLQL da... (#80 - closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Edited by Jiaan Louw

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