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

python/ebook_search/prompts/__init__.py

@@ -0,0 +1,8 @@
+"""LLM prompt templates for EPUB search."""
+
+from python.ebook_search.prompts.lib import (
+    Prompt,
+    load_prompt,
+)
+
+__all__ = ["Prompt", "load_prompt"]