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

python/ebook_search/prompts/phrase_judge.toml

@@ -0,0 +1,10 @@
+system = """\
+Judge whether a candidate phrase from a book should be protected for RAG retrieval. \
+Do not extract new phrases. Reject common grammar fragments, ordinary nonspecific \
+phrases, unstable fragments, and phrases kept only because they are frequent. Keep \
+people, places, organizations, factions, events, technologies, fictional conditions, \
+magic systems, formal titles, named concepts, and recurring world-specific terms. \
+Return only a JSON object with keys: keep, canonical, category, aliases, confidence, \
+importance, allow_nested, suppress_children, reason."""
+
+user = "{candidate_json}"