feat: Extract versioned core/editor packages (closes #77)

[lennythebadass]

What this does

This implements the ADR-0029 compatibility-preserving package split while keeping the existing starry-slides user experience intact.

Changes

• Adds workspace package boundaries for @starrykit/slides-core and @starrykit/slides-editor.
• Moves reusable slide-domain logic into slides-core with public exports for contracts, document/history operations, validation helpers, and export planning.
• Moves the React editor into slides-editor and exposes a host-driven SlidesEditor component with props/callbacks.
• Keeps the root starry-slides package as the compatibility/runtime entrypoint for CLI, Skill usage, local preview, local rendering, and exports.
• Adds compatibility re-exports under the root source paths so existing internal CLI/runtime behavior keeps working.
• Keeps Skill packaging in the root package files list.
• Adds package-boundary tests to guard the new public surfaces and the root CLI/Skill compatibility surface.

Verification

• pnpm install --lockfile-only
• pnpm lint
• pnpm test
• pnpm build
• pnpm test:packaged-cli

Closes #77

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[Mine77]

是否影响现有npm publish的ci流程？

[Mine77]

@lennythebadass

[Mine77]

@lennythebadass 请根据这个评论做一个小修改并 push 到这个 PR。