* fix: remove broken relative import in rag_engine example
The snippet imported from a non-existent .prompts module and used an
undefined return_instructions_root(). Replaced with a self-contained
instruction string so the knowledge-engine example runs standalone.
* Apply suggestion from @joefernandez
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Co-authored-by: Joe Fernandez <931947+joefernandez@users.noreply.github.com>