fix: ship external-fix script + lychee JSON save so make external-fix works fresh

scripts/fix_dead_external_links.py was referenced by the Makefile but never
committed, and check_external_links.py did not yet save linkcheck/external.json
(both were working-tree only), so external-fix/external-fix-apply failed on
any fresh clone. The fixer is deliberately conservative: only DNS-failed /
connection-failed / host-unreachable links are auto-commented; .onion links
and ambiguous failures (404/5xx/SSL/refused) go to manual review.

scripts/check_external_links.py

@@ -89,6 +89,10 @@ def main():
         for src, line, url, status in sorted(dead):
             f.write(f"{src}:{line}\t{url}\t[{status}]\n")
 
+    # Save the full lychee JSON so `make external-fix` can act on this run's data
+    # without re-pinging the web (see scripts/fix_dead_external_links.py).
+    (outdir / "external.json").write_text(json.dumps(data, indent=2))
+
     print(f"External link check: {total_checked} links checked, {ok} ok, {len(dead)} dead.")
     if dead:
         print("\nDead external links:")