Address PR #87 feedback: Enhance documentation and rename to AGENTS.md

[candour]

This change addresses the feedback from @gundalow and @fluffle on PR fluffle#87. It renames codebase.md to AGENTS.md, incorporates key architectural details from the Wiki (Directory Structure, Event System, and Rewriters), and adds specific guidance on testing best practices, global bot safety, and architectural warnings for pollers.

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PR created automatically by Jules for task 8138048201386111517 started by @candour

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[gundalow]

Thanks for this.
Structure looks good, thanks for using ### and not bold for headings.

I let fluffle do the technical review.

Once Mongodb has been fully removed we should update the dual write section.