feat(root-cms): add array item defaults via itemDefault

[stevenle]

Motivation

• Provide schema authors a way to initialize newly added array items with a configured default object so CMS users get sensible starting values.
• Ensure new array items are independent objects (cloned) to avoid shared references.

Description

• Add a new itemDefault?: Record<string, any> option to ArrayField in packages/root-cms/core/schema.ts so schema.array() can accept an item default.
• Update the DocEditor array logic in packages/root-cms/ui/components/DocEditor/DocEditor.tsx to accept and propagate a defaultValue for array actions and to apply structuredClone(action.defaultValue || {}) when inserting new items for add, insertBefore, and insertAfter so each item receives its own cloned default object.
• Extend the TypeScript action interfaces (ArrayAdd, ArrayInsertBefore, ArrayInsertAfter) to carry the defaultValue and pass field.itemDefault || {} when dispatching those actions.
• Add a changeset for @blinkk/root-cms describing the feature.

Testing

• No automated tests were executed for this change (tests were intentionally not run per instructions).

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Codex Task