feat(lib): vendor-skills helper, five MengTo scroll skills pinned

lib/vendor-skills.sh: vendor_pinned_skills <lock-key> [refresh], list or
dict lock shapes, lock read via python argv, traversal and charset guard
on lock values, VENDOR_BASE_URL honoured only as file:// (hermetic suite),
per-file tmp+mv, refresh skips a skill never installed. install-plugins.sh
Step 8e and update-all.sh 7.3 call it for agent-skills and mengto-skills.
Vendored at a965851: scroll-world-storytelling, build-threejs-scroll-worlds
(+5 references), scroll-scrubbed-visual-sequence, scroll-scrubbed-word-
reveal, scroll-progress-timeline; text files only. Registered in link.sh,
.gitignore, toggle-external, profile.sh and the design/web/web-full/full
profiles, which also list site-motion (personal). Suite: 8 cases.

lib/profiles/web.profile

@@ -38,6 +38,16 @@ frontend-design                   external
 design-motion-principles          external
 impeccable                        external
 
+# External: Mengto scroll-choreography skills (curl, commit-pinned)
+scroll-world-storytelling         external
+build-threejs-scroll-worlds       external
+scroll-scrubbed-visual-sequence   external
+scroll-scrubbed-word-reveal       external
+scroll-progress-timeline          external
+
+# Personal: motion implementation companion (skills/site-motion)
+site-motion                       personal
+
 # External: 21st.dev pack (publishing flows 21st-registry / -design-sync
 # stay parked)
 21st-ui-build                     external