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

openclaw.plugin.json

@@ -65,6 +65,11 @@
             "type": "integer",
             "minimum": 0
           },
+          "max_section_chars": {
+            "type": "integer",
+            "minimum": 1,
+            "description": "Max chars per section before splitting into sub-chunks. Default 4000."
+          },
           "file_patterns": {
             "type": "array",
             "items": {