feat: RAG answers with sutta citations (M1)

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

src/buddhagpt/rag.py

@@ -0,0 +1,25 @@
+from pathlib import Path
+from buddhagpt.index import search
+
+SYSTEM = (
+    "You are a thoughtful guide grounded in the Pali Canon. Answer with warmth and "
+    "precision. Base doctrinal claims on the provided passages and cite them by uid "
+    "(e.g. [mn21]). If the passages do not cover the question, say so plainly."
+)
+
+def build_prompt(question: str, passages: list[dict]) -> list[dict]:
+    ctx = "\n\n".join(f"[{p['uid']}] {p['title']}:\n{p['chunk']}" for p in passages)
+    return [
+        {"role": "system", "content": SYSTEM},
+        {"role": "user", "content": f"Passages:\n{ctx}\n\nQuestion: {question}"},
+    ]
+
+def answer(question: str, model_path: str, db_path: Path) -> dict:
+    from mlx_lm import load, generate
+    passages = search(db_path, question, k=4)
+    model, tokenizer = load(model_path)
+    prompt = tokenizer.apply_chat_template(
+        build_prompt(question, passages), tokenize=False, add_generation_prompt=True
+    )
+    text = generate(model, tokenizer, prompt=prompt, max_tokens=500)
+    return {"answer": text, "citations": [{"uid": p["uid"], "title": p["title"]} for p in passages]}