Refactor notebooks for direct Colab editing

[Laverde97]

Summary

Refactors the notebook generation workflow so Colab/Gemini edits can land directly in notebooks/*.ipynb without being retyped into scripts/content.py.

Closes #43.

What changed

• turned scripts/gen_notebooks.py into an in-place, idempotent notebook normalizer
• regenerated only the centrally owned scaffold: header, Setup preamble, Setup code, and footer
• preserved notebook-owned teaching body cells in place
• stripped outputs, execution counts, and transient per-cell Colab metadata (colab, outputId, executionInfo)
• preserved folded-solution metadata (cellView: "form", jupyter.source_hidden, and tags: ["solution", "hide-input"])
• preserved existing valid cell IDs; missing/invalid/duplicate IDs receive stable content-derived IDs
• moved section objectives into _variables.yml as bilingual objectives_en / objectives_es
• reduced scripts/content.py to centrally owned Setup code only
• updated CLAUDE.md and notebooks/README.md to document the new ownership boundary and Colab → GitHub workflow

Validation

• baseline run on untouched notebooks produced zero .ipynb diff
• ran the normalizer twice with no further notebook changes
• nbformat.validate: 12/12 valid
• verified all 12 Setup cells are unchanged from the pre-refactor versions
• simulated a Colab/Gemini edit and verified:
  • teaching body source is preserved
  • outputs and execution counts are removed
  • transient Colab metadata is removed
  • folded-solution metadata is preserved
  • new cells receive stable IDs
• uv run --with pyyaml,nbformat python scripts/check_links.py → All checks passed
• git diff --check → clean
• git diff -- 'notebooks/*.ipynb' → empty
• rendered successfully with repository-pinned Quarto 1.6.40

Scope

This PR changes notebook infrastructure and ownership only. It does not implement the pedagogical notebook changes planned for #44 and does not modify the teaching bodies of the tracked notebooks.

[project-delphi]

LGTM.

[project-delphi]

We can reopen if this blocks the notebooks being updated.