lecture-python.myst: HTML-only PR previews; three-format build behind a full-preview label

Phase 1 critical path of #92 — this item alone outweighs every actions-repo item there combined. Work lands in QuantEcon/lecture-python.myst; expected saving −7–7.8 min against its measured 17-minute PR baseline.

Its .github/workflows/ci.yml still runs the notebook build (--custom-builder=jupyter, :72) and pdflatex (:84) on every PR, ungated — formats unused by a preview.

• HTML-only build on every PR
• Three-format build behind a full-preview label
• PDF/notebook coverage moves to the weekly cache.yml
• fetch-depth: 1 on the checkout (ci.yml:21, still fetch-depth: 0) — −80–95s measured; land together with the bounded-fetch change in the actions-repo quick-wins PR (#147), since a shallow checkout without bounded fetches makes them worse

Gated on the preview-SLA decision (what happens to the PDF/ipynb download buttons), not on effort — see the blocking issue #146.

Start with .github/workflows/ci.yml, especially the notebook build at line 72, pdflatex step at line 84, and checkout at line 21; compare the weekly cache.yml workflow. Confirm the preview-SLA decision in blocking issue #146 and coordinate with quick-wins PR #147. Done means PRs build HTML only, full three-format builds require the full-preview label, PDF/notebook coverage moves to cache.yml, and checkout uses fetch-depth: 1.