Ignore retired O'Reilly webinar link in link checker

The README links to a June 2017 O'Reilly webinar whose events microsite has since been retired, so the page now returns 403 and fails the PR link checker on every unrelated README change. Add it to .lycheeignore alongside the other kept-for-reference dead links so the link stays visible in the README without breaking CI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EDwBfCivCNdKJA2BexAizd

Author: rrrutledge

.lycheeignore

@@ -14,4 +14,6 @@ https://www.linkedin.com/in
 # from source-code-inventory.md / no longer reachable but we want to keep the link in the pattern in case we find it again.
 https://github.com/trieshard/source-strategy-assessment/blob/master/framework.md
 https://dl.acm.org/doi/10.5555/3158161.3158175
-https://retromat.org/en/
+https://retromat.org/en/
+# O'Reilly retired its events microsite, so this 2017 webinar page now returns 403; the link stays in the README as a record of the session.
+http://www.oreilly.com/pub/e/3884