Add plugin update workaround to README install section

Claude Code's /plugin update doesn't refresh the local cache, a known
issue with 10+ reports (anthropics/claude-code#36317). Added NOTE with
cache-clearing workaround for users stuck on old versions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aklofas

README.md

@@ -92,6 +92,15 @@ For the end-to-end walkthrough from S-expression parsing through signal detectio
 /plugin install kicad-happy@kicad-happy
 ```
 
+> [!NOTE]
+> `/plugin update` may not detect new versions due to a [known Claude Code issue](https://github.com/anthropics/claude-code/issues/36317).
+> To get the latest version, clear the cache and reinstall:
+> ```
+> rm -rf ~/.claude/plugins/cache/kicad-happy ~/.claude/plugins/marketplaces/kicad-happy
+> /plugin marketplace add aklofas/kicad-happy
+> /plugin install kicad-happy@kicad-happy
+> ```
+
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 Clone and open the repo in Codex — skills are auto-discovered from `.agents/skills/`: