Skip O'Reilly webinar link in the PR link checker

O'Reilly returns 403 to lychee's automated requests, so the June 2017 webinar link fails the PR link checker even though the page loads fine in a browser. Keep the link in the README and add it to .lycheeignore so CI stops flagging it.

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 returns 403 to the link checker's automated requests, though the page loads fine in a browser.
+http://www.oreilly.com/pub/e/3884