feat: add master notebook library scaffolding and review tooling

Add CLAUDE.md defining the Palladium master notebook conventions and
Red Panda Approval criteria, plus a review-notebook slash command for
LLM-driven notebook review.

Expand .gitignore to block client/engagement documents and generated
exports, keeping masters client-clean while allowing text/image sources.

Normalize slider widget numeric values from floats to integers in
notebook JSON.

Makefile

@@ -4,7 +4,7 @@ VENV    := .venv
 PY      := $(VENV)/bin/python
 PIP     := $(VENV)/bin/pip
 
-.PHONY: setup lab test lint format clean
+.PHONY: setup lab test check-notebooks lint format clean
 
 ## One-time: create venv, install deps + palladium (editable)
 setup:
@@ -19,10 +19,16 @@ setup:
 lab:
 	$(VENV)/bin/jupyter lab
 
-## Run the test suite (no Athena connection needed — HTTP is mocked)
+## Run the test suite (no Athena connection needed — HTTP is mocked).
+## Includes the notebook structural suite (tests/test_notebooks.py).
 test:
 	$(PY) -m pytest tests/ -v
 
+## Structural checks for every master notebook (kernel-free; -rs shows
+## why grandfathered notebooks skip the notebook-first tier)
+check-notebooks:
+	$(PY) -m pytest tests/test_notebooks.py -rs
+
 lint:
 	$(VENV)/bin/ruff check .