add emil-design-eng skill to install/update pipeline

- Download SKILL.md from emilkowalski/skill to skills-external/
- Symlink to ~/.claude/skills/ via link.sh
- install-plugins.sh step 9: curl download + symlink check
- update-all.sh step 8: re-fetch latest SKILL.md on update
- plugins.lock.json: add emil-design-eng entry
- .gitignore: ignore auto-created symlink in skills/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -41,6 +41,9 @@ skills/setup-deploy
 skills/ship
 skills/unfreeze
 
+# Emil Design Engineering skill symlink — auto-created by link.sh
+skills/emil-design-eng
+
 # Local project config (per-machine, not shared)
 .claude/