feat: use native Confluence task list macros

[geopanther]

Summary

Replace HTML <input> checkboxes with native Confluence ac:task-list/ac:task macros for task list rendering. Confluence strips <input> elements, causing task lists to render without checkboxes.

Changes

• Override task_list_item() in ConfluenceRenderer to emit ac:task with ac:task-id, ac:task-status (complete/incomplete), and ac:task-body
• Override list() to wrap task items in ac:task-list instead of <ul>
• Update README plugin table: task lists now documented as "Rendered as native Confluence task list"
• Update test assertion to check for ac:task-list instead of task-list-item-checkbox

Tests performed

• All 140 unit tests pass
• Uploaded test page to Confluence and verified task list checkboxes render correctly with proper complete/incomplete status