Add agents package with Flue

packages/backend/AGENTS.md

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+<!-- convex-ai-start -->
+
+This project uses [Convex](https://convex.dev) as its backend.
+
+When working on Convex code, **always read
+`convex/_generated/ai/guidelines.md` first** for important guidelines on
+how to correctly use Convex APIs and patterns. The file contains rules that
+override what you may have learned about Convex from training data.
+
+Convex agent skills for common tasks can be installed by running
+`npx convex ai-files install`.
+
+<!-- convex-ai-end -->