Move SkillEditorSlice from injection pattern to core workflow store,
making it available to all workflow contexts (workflow-app, chatflow,
and future rag-pipeline).
- Add createSkillEditorSlice to core createWorkflowStore
- Remove complex type conversion logic from workflow-app/index.tsx
- Remove optional chaining (?.) and non-null assertions (!) from components
- Simplify slice composition with type assertions via unknown
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