chore(config): untrack emil-design-eng skill (machine-owned curl copy)

skills-external/emil-design-eng/SKILL.md is curl'd from emilkowalski/skill
by install-plugins.sh when absent and re-fetched unconditionally by every
update-all.sh run, so tracking it produced a repo diff on each upstream edit
(latest: Radix vars dropped for Base UI). Same category as frontend-design/
and impeccable/, already ignored on that rationale — a fresh clone re-fetches
it, so no offline copy is needed and nothing was pinned here anyway.

design-motion-principles/ has the same overwrite-on-update behaviour but NOT
the same bootstrap: install-plugins.sh only warns instead of cloning it, so it
stays tracked until that gap is closed.

plugins.lock.json

@@ -36,7 +36,7 @@
     "source": "https://github.com/emilkowalski/skill",
     "path": "skills/emil-design-eng/SKILL.md",
     "managed_by": "curl",
-    "note": "Emil Kowalski's design engineering skill — UI polish, animations, component craft. Downloaded to skills-external/emil-design-eng/, symlinked by link.sh."
+    "note": "Emil Kowalski's design engineering skill — UI polish, animations, component craft. Machine-owned: curl'd to skills-external/emil-design-eng/ (gitignored, re-fetched by update-all.sh), symlinked by link.sh."
   },
   "impeccable": {
     "source": "npm:impeccable",