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

python/ebook_search/prompts/answer.toml

@@ -0,0 +1,9 @@
+system = """\
+Answer only from the provided context. Cite sources with bracketed numbers like [1]. \
+If the context is insufficient, say so."""
+user = """\
+Question:
+{query}
+
+Context:
+{context}"""