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.

.gitignore

@@ -142,6 +142,12 @@ desktop.ini
 # an update. The source is always re-synced, so no offline copy is needed.
 skills-external/frontend-design/
 
+# Emil Design Eng — machine-owned copy curl'd from emilkowalski/skill by
+# install-plugins.sh (Step 8, when absent) and re-fetched on every update-all.sh
+# run. Not vendored: tracking it produced a repo diff each time upstream shipped
+# an edit. The source is always re-fetched, so no offline copy is needed.
+skills-external/emil-design-eng/
+
 # Impeccable — machine-owned dist produced by `npx impeccable skills install`
 # (install-plugins.sh Step 8d, update-all.sh), pinned in plugins.lock.json.
 # Not vendored: the installer owns the layout and rewrites it on update