skills(cutover): hide the objectui bundle from the root skills CLI until it moves into skills/ (bill item 18, objectstack#25803)

create-objectstack's skills-catalog boundary test lists every tracked
SKILL.md and exempts only the root skills/ catalog, so ui/skills/objectui
must either move there or carry `metadata.internal: true`. The move
(decision C) is its own card after the import; until then the bundle is
marked internal, which changes nothing a customer sees: the root CLI
never listed it before the import either.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RSDL3SFtJaKzcymvCD8RsU

Author: claude

ui/skills/objectui/SKILL.md

@@ -4,6 +4,7 @@ description: Universal Server-Driven UI (SDUI) engine for building JSON-driven R
 license: Apache-2.0
 compatibility: Tracks the @objectstack/spec range each @object-ui/* package declares in its own manifest; read it there. This file states no version literal, per the doc-version-claims ratchet.
 metadata:
+  internal: true
   author: objectstack-ai
   version: "2.0"
   domain: ui