Collect stars and last push during the weekly audit

The audit already asks GitHub about every linked repository, so the same
query can carry back a star count and a last-push date for free. Results go
to repo_meta.json, which lets a search result say which of two hundred
linked PoCs is worth opening.

2.1 MB for 45,008 repositories, refreshed weekly.

.github/workflows/audit_poc_links.yml

@@ -39,6 +39,7 @@ jobs:
           git add --all -- \
             .github/link_audit_state.json \
             github.txt \
+            repo_meta.json \
             ':(top,glob)[12][0-9][0-9][0-9]/CVE-*.md'
 
           if git diff --cached --quiet; then