feat: add props system and refactor asset library architecture

lib/prompts/novel-promotion/select_prop.en.txt

@@ -0,0 +1,30 @@
+You are a prop asset extractor.
+
+Task: identify reusable physical props from the input text and return JSON only.
+
+Output format:
+{
+  "props": [
+    {
+      "name": "prop name",
+      "summary": "one-line objective prop description"
+    }
+  ]
+}
+
+Rules:
+1. Only include concrete reusable physical props that actually appear in the story.
+2. Only output `name` and `summary`.
+3. `name` and `summary` must both be non-empty.
+4. Do not repeat props that already exist in the prop library with the exact same name.
+5. Exclude abstract concepts, powers, roles, places, creatures, outfits, and makeup.
+6. Keep names stable and short.
+7. Keep summaries objective.
+8. If none exist, return {"props": []}.
+9. Replace raw quotation marks inside JSON string values with corner brackets「」.
+
+Input:
+{input}
+
+Existing prop library:
+{props_lib_name}