feat(prompts): implement TOML-backed prompt loading and refactor message generation

tests/ebook_search/test_http.py

@@ -93,6 +93,14 @@ async def test_answer_query_uses_httpx_chat_completions(mocker: MockerFixture) -
     payload = kwargs["json"]
     assert isinstance(payload, dict)
     assert payload["model"] == "deepseek-v4-flash"
+    assert payload["messages"] == [
+        {
+            "role": "system",
+            "content": "Answer only from the provided context. Cite sources with bracketed numbers like [1]. "
+            "If the context is insufficient, say so.",
+        },
+        {"role": "user", "content": "Question:\nquestion\n\nContext:\n[1] Book\nsource"},
+    ]
 
 
 async def test_embed_texts_uses_httpx_embeddings(mocker: MockerFixture) -> None: