remove notebooks

- Delete notebooks/ (personal scratch artifacts; gitignored)
- Move nbformat to --extra migrate; lazy imports in migrate.py and
  cli.py so base CLI loads without it
- Restore langchain-text-splitters in core (chunking dependency)

pyproject.toml

@@ -24,15 +24,12 @@ dependencies = [
     "numpy>=1.26.0; python_version >= '3.12'",
     "pytesseract>=0.3.10",
 
-    # Text processing and notebook migration
-    "langchain-text-splitters>=0.0.2",
-    "nbformat>=5.9.0",
-
     # File watching
     "watchdog>=3.0.0",
 
     # Utilities
     "platformdirs>=4.2.0",
+    "langchain-text-splitters>=0.0.2",
 ]
 
 [project.optional-dependencies]
@@ -72,6 +69,9 @@ kg = [
 neo4j = [
     "neo4j>=5.0",
 ]
+migrate = [
+    "nbformat>=5.9.0",
+]
 grobid = []
 full = [
     "ocr-pipeline[paddle,tables,figures,scientific]",