chore(gitignore): ignore new gstack skills from v0.17.0 submodule bump

gstack 0.17.0 added 5 skills that land in skills/ via gstack ./setup
(benchmark-models, context-restore, context-save, make-pdf, plan-tune).
The older gstack skills are already ignored here — extend the list so
these don't show up as untracked after `make update`.

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

.gitignore

@@ -40,6 +40,11 @@ skills/setup-browser-cookies
 skills/setup-deploy
 skills/ship
 skills/unfreeze
+skills/benchmark-models
+skills/context-restore
+skills/context-save
+skills/make-pdf
+skills/plan-tune
 
 # Emil Design Engineering skill symlink — auto-created by link.sh
 skills/emil-design-eng