feat(indexer): hierarchical chunking for large sections

- Section-split first for structured notes
- Large sections (>max_section_chars) broken via sliding-window
- Small sections stay intact with heading preserved
- Adds max_section_chars config (default 4000)
- 2 new TDD tests for hierarchical chunking

python/obsidian_rag/config.py

@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import json
-import os
 from enum import Enum
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -32,6 +31,7 @@ class VectorStoreConfig:
 class IndexingConfig:
     chunk_size: int = 500
     chunk_overlap: int = 100
+    max_section_chars: int = 4000
     file_patterns: list[str] = field(default_factory=lambda: ["*.md"])
     deny_dirs: list[str] = field(
         default_factory=lambda: [