CI has no build cache to restore — every PR's preview fails at the cache-download step

Found while landing #12 (2026-08-19): the preview job failed with ##[error]no matching workflow run found with any artifacts at its dawidd6 cache-download step, and the cause is structural — this repo has no cache.yml workflow at all (workflows present: ci/publish only), so there is never a build-cache artifact to restore. Every PR's CI is red for this pre-existing reason regardless of the change (the Netlify deploy-preview itself built fine on #12), which trains reviewers to merge over a red check.

Also downstream of this: the site cannot surface #12's fixes (the dataBHS repoint + usetex guards in the downloadable five_preferences.ipynb) until a publish succeeds, so the publish path needs the same look.

Options, roughly in order: add a cache.yml on the family pattern (weekly cron + dispatch, upload _build as build-cache); or make the cache download tolerant (if_no_artifact_found: warn) so CI builds from scratch — slower but honest; or wait for the QuantEcon/actions migration, where restore-jupyter-cache handles the missing-cache case — see QuantEcon/actions#160 for the cache-mode decision that migration should land with.

🤖 Generated with Claude Code

Start by comparing the existing ci and publish workflows with the missing cache.yml workflow, then inspect the dawidd6 cache-download step and the referenced QuantEcon/actions#160 decision. The work is done when preview CI no longer fails solely because no build-cache artifact exists and the publish path can succeed.