restore-jupyter-cache: cache freshness warning and cache-age-days output

[mmcky]

#92 Phase 2: cache freshness warning in restore-jupyter-cache, so a stale cache is loud instead of silent (the #83 failure mode: 40-minute cold builds on every PR for ~2 months).

• Write .cache-metadata.json at save time
• Age check on restore: ::warning:: + step-summary when the cache is older than expected
• New cache-age-days output

Not started — the action declares exactly two outputs, cache-hit and cache-key (restore-jupyter-cache/action.yml:39-45), and no cache-metadata/cache-age string exists in the repo. No consumer repin needed on release: all 11 call sites float on @v0.