feat: RAG answers with sutta citations (M1)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_rag.py

@@ -0,0 +1,10 @@
+from buddhagpt.rag import build_prompt
+
+def test_build_prompt_includes_passages_and_citation_instruction():
+    msgs = build_prompt("What causes suffering?", [
+        {"uid": "sn56.11", "title": "Setting the Wheel in Motion", "chunk": "Craving leads to suffering."}
+    ])
+    system = msgs[0]["content"]
+    assert "sn56.11" in msgs[1]["content"]
+    assert "cite" in system.lower()
+    assert msgs[1]["content"].endswith("What causes suffering?")