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/tests/unit/test_chunker.py

@@ -206,6 +206,7 @@ def _mock_config(tmp_path: Path) -> MagicMock:
     cfg.vault_path = str(tmp_path)
     cfg.indexing.chunk_size = 500
     cfg.indexing.chunk_overlap = 100
+    cfg.indexing.max_section_chars = 4000
     cfg.indexing.file_patterns = ["*.md"]
     cfg.indexing.deny_dirs = [".obsidian", ".trash", "zzz-Archive", ".git"]
     cfg.indexing.allow_dirs = []
@@ -248,3 +249,41 @@ def test_chunk_file_unstructured(tmp_path: Path):
     assert len(chunks) > 1
     assert all(c.section is None for c in chunks)
     assert chunks[0].chunk_index == 0
+
+
+def test_large_section_split_into_sub_chunks(tmp_path: Path):
+    """Large section (exceeding max_section_chars) is split via sliding window."""
+    vault = tmp_path / "Notes"
+    vault.mkdir()
+    fpath = vault / "2024-03-15-Podcast.md"
+    large_content = "word " * 3000  # ~15000 chars, exceeds MAX_SECTION_CHARS
+    fpath.write_text(f"# Episode Notes\n\n{large_content}")
+
+    cfg = _mock_config(tmp_path)
+    cfg.indexing.max_section_chars = 4000
+    chunks = chunk_file(fpath, fpath.read_text(), "2024-03-15T10:00:00Z", cfg)
+
+    # Large section should be split into multiple sub-chunks
+    assert len(chunks) > 1
+    # Each sub-chunk should preserve the section heading
+    for chunk in chunks:
+        assert chunk.section == "#Episode Notes", (
+            f"Expected #Episode Notes, got {chunk.section}"
+        )
+
+
+def test_small_section_kept_intact(tmp_path: Path):
+    """Small section (under max_section_chars) remains a single chunk."""
+    vault = tmp_path / "Notes"
+    vault.mkdir()
+    fpath = vault / "2024-03-15-Short.md"
+    fpath.write_text("# Notes\n\nShort content here.")
+
+    cfg = _mock_config(tmp_path)
+    cfg.indexing.max_section_chars = 4000
+    chunks = chunk_file(fpath, fpath.read_text(), "2024-03-15T10:00:00Z", cfg)
+
+    # Small section → single chunk
+    assert len(chunks) == 1
+    assert chunks[0].section == "#Notes"
+    assert chunks[0].text.strip().endswith("Short content here.")