Feat: add toc extraction method supported by llm

rag/prompts/assign_toc_levels.md

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+
+You are given a JSON array of TOC items. Each item has at least {"title": string} and may include an existing structure.
+
+Task
+- For each item, assign a depth label using Arabic numerals only: top-level = 1, second-level = 2, third-level = 3, etc.
+- Multiple items may share the same depth (e.g., many 1s, many 2s).
+- Do not use dotted numbering (no 1.1/1.2). Use a single digit string per item indicating its depth only.
+- Preserve the original item order exactly. Do not insert, delete, or reorder.
+- Decide levels yourself to keep a coherent hierarchy. Keep peers at the same depth.
+
+Output
+- Return a valid JSON array only (no extra text).
+- Each element must be {"structure": "1|2|3", "title": <original title string>}.
+- title must be the original title string.
+
+Examples
+
+Example A (chapters with sections)
+Input:
+["Chapter 1 Methods", "Section 1 Definition", "Section 2 Process", "Chapter 2 Experiment"]
+
+Output:
+[
+  {"structure":"1","title":"Chapter 1 Methods"},
+  {"structure":"2","title":"Section 1 Definition"},
+  {"structure":"2","title":"Section 2 Process"},
+  {"structure":"1","title":"Chapter 2 Experiment"}
+]
+
+Example B (parts with chapters)
+Input:
+["Part I Theory", "Chapter 1 Basics", "Chapter 2 Methods", "Part II Applications", "Chapter 3 Case Studies"]
+
+Output:
+[
+  {"structure":"1","title":"Part I Theory"},
+  {"structure":"2","title":"Chapter 1 Basics"},
+  {"structure":"2","title":"Chapter 2 Methods"},
+  {"structure":"1","title":"Part II Applications"},
+  {"structure":"2","title":"Chapter 3 Case Studies"}
+]
+
+Example C (plain headings)
+Input:
+["Introduction", "Background and Motivation", "Related Work", "Methodology", "Evaluation"]
+
+Output:
+[
+  {"structure":"1","title":"Introduction"},
+  {"structure":"2","title":"Background and Motivation"},
+  {"structure":"2","title":"Related Work"},
+  {"structure":"1","title":"Methodology"},
+  {"structure":"1","title":"Evaluation"}
+]