refactor: move task updates into document session

[PathGao]

Summary

• move task-checkbox Markdown mutation and persistence into documentSession
• keep DOM checkbox discovery and visual state in MarkdownViewer
• preserve the in-memory update before save invariant

Depends on #275. Merge after #275. Watcher and close-confirmation ownership remain subsequent focused steps.

Validation

• npm run check
• npm test (118 passing)
• cargo test (20 passing on the rebased parent)

[PathGao]

@alecdotdev This is the next dependent document-session PR after #275. It isolates task-content mutation while retaining DOM-specific behavior in MarkdownViewer.