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

[mmcky]

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 (feat: add save-cache option for PR-scoped build caching #24), the save-mode outputs/fail-on-miss bugs were fixed by fix(restore-jupyter-cache): save-mode outputs and fail-on-miss are broken #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 (Phase 1 quick wins: bounded SHA fetches, concurrency-group fallback, paths-ignore in templates/ci.yml #147).