skills(objectstack-ui): state the one-app-per-package cap in App Navigation

The App Navigation section tells an author to `register under
defineStack({ apps: [...] })` and stops there. Nothing in the published
bundle says the array holds one entry: measured across `skills/`, the
one-app rule appears 0 times in any spelling, while `defineStack` appears
54 times and `App.create` 3 — the zeros are readings, not an empty corpus.
An agent authoring `*.app.ts` from this skill therefore has no reason not
to write one app per audience, and finds out at `defineStack` load time,
after the design is written.

All three `ADR-0019` citations in the published bundle point at
`0019-approval-as-flow-node`, so this one names its record explicitly.

⚠️ This overruns the published-skills token ratchet by 59 tokens
(3874 against a ceiling of 3815, headroom measured at 0 before writing).
The ceiling is deliberately NOT touched and nothing else is trimmed to
pay for it — both are maintainer decisions. See the PR body.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012GKcPZbMoGq7WPzKLfRBTU

Author: claude

skills/objectstack-ui/SKILL.md

@@ -160,6 +160,10 @@ An **App** groups objects, dashboards, reports, and custom pages into a
 structured navigation tree. Build with `App.create({...})` from
 `@objectstack/spec/ui` and register under `defineStack({ apps: [...] })`.
 
+⛔ **Exactly one App per `type: 'app'` package** — `defineStack` refuses a
+second at load, per ADR-0019 (app-as-consumer-unit) D3. More audiences are
+`requiredPermissions`-gated groups **inside** that one app, never a second app.
+
 ### Navigation Item Types
 
 | Type | Properties | Purpose |