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.

.claude/commands/review-notebook.md

@@ -0,0 +1,13 @@
+---
+description: LLM review of a master notebook (content, cross-cell, staging, tags)
+argument-hint: <path/to/notebook.ipynb>
+---
+
+Read `docs/Notebook_Review_Prompt_V1-00.md` (repo root: the Palladium repository
+this command lives in) and apply it, in full, to the notebook at: $ARGUMENTS
+
+Follow the prompt exactly: read the pattern docs it names, read the notebook's raw
+`.ipynb` JSON (cell sources, tags, and outputs — not a rendered view), and open the
+master's engine package for cross-checks where the prompt directs. Produce ONLY the
+findings report in the prompt's output format — no preamble, no summary of what you
+did.