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

AGENTS.md

@@ -42,6 +42,11 @@ Plugin `package.json` MUST have:
 
 User config at `~/.obsidian-rag/config.json` or `./obsidian-rag/` dev config.
 
+Key indexing fields:
+- `indexing.chunk_size` — sliding window chunk size (default 500)
+- `indexing.chunk_overlap` — overlap between chunks (default 100)
+- `indexing.max_section_chars` — max chars per section before hierarchical split (default 4000)
+
 Key security fields:
 - `security.require_confirmation_for` — list of categories (e.g. `["health", "financial_debt"]`). Empty list disables guard.
 - `security.auto_approve_sensitive` — `true` bypasses sensitive content prompts.
@@ -49,7 +54,11 @@ Key security fields:
 
 ## Ollama Context Length
 
-`python/obsidian_rag/embedder.py` truncates chunks at `MAX_CHUNK_CHARS = 8000` before embedding. If Ollama 500 error returns, increase this value or reduce `indexing.chunk_size` in config.
+`python/obsidian_rag/embedder.py` truncates chunks at `MAX_CHUNK_CHARS = 8000` before embedding. If Ollama 500 error returns, increase `max_section_chars` (to reduce section sizes) or reduce `chunk_size` in config.
+
+## Hierarchical Chunking
+
+Structured notes (date-named files) use section-split first, then sliding-window within sections that exceed `max_section_chars`. Small sections stay intact; large sections are broken into sub-chunks with the parent section heading preserved.
 
 ## Sensitive Content Guard