chore(skills): drop find-skills — unused, and its update step began timing out

RC fix (soak day 1): `make update` step 7.5 failed on a 300s clone timeout
of alchaincyf/find-skills. The skill (search the skills.sh registry from
Claude) was never used; the discovery case stays reachable manually via
`npx -y skills find <query>`. Removed from install-plugins.sh (install list
+ summary + comment), update-all.sh (refresh list), link.sh
(NPX_EXTERNAL_SKILLS), lib/toggle-external.sh (MANAGED_TOOLS + case arms),
plugin-advisor.md, .gitignore; local skills/find-skills symlink deleted.
Memory-registry and test-fixture mentions kept — append-only history.
toggle-external `list` verified post-removal; suites 8/8.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XpphkdTosUzokBDNG7PToS

link.sh

@@ -90,7 +90,7 @@ done
 # absolute paths so the link stays valid regardless of where the
 # repo is cloned (relative ../../ paths broke on repos deeper than
 # one level below $HOME).
-NPX_EXTERNAL_SKILLS=(darwin-skill find-skills)
+NPX_EXTERNAL_SKILLS=(darwin-skill)
 for _ext in "${NPX_EXTERNAL_SKILLS[@]}"; do
   _target="$HOME/.agents/skills/$_ext"
   _link="$REPO/skills/$_ext"