lecture-dp: save-cache one-liner, cache-on-merge job, shallow checkout

The lecture-dp consumer items from #92 Phase 1. Work lands in QuantEcon/lecture-dp.

• save-cache: 'true' on the restore step (one line). Its ci.yml still calls quantecon/actions/restore-jupyter-cache@v0 without it. The input exists since v0.5.2 (#24), the save-mode outputs/fail-on-miss bugs were fixed by #104 in v0.9.0, and the path is CI-covered since v0.10.0 — nothing blocks it.
• Cache-on-merge job on push to main (paths-ignore: [environment.yml]; the weekly rebuild stays the anchor; no actions-repo change). Its cache.yml triggers on the weekly cron plus push: paths: [environment.yml] — the inverse of what this needs.
• fetch-depth: 1 on the ci.yml:19 checkout (still fetch-depth: 0) — land together with the bounded-fetch change in the actions-repo quick-wins PR (#147).

Start in the lecture-dp consumer's ci.yml, including checkout line 19, and its cache.yml; compare the requested cache input, push trigger, and fetch depth with the existing workflows. Review the actions-repo quick-wins PR #147 before changing the checkout setting. Done means the restore step saves the cache, main pushes trigger the cache job while ignoring environment.yml, and checkout uses depth 1.