feat: task list support in both directions

[blaipr]

Problem

Markdown checkbox lists (- [ ] / - [x]) uploaded as literal bracket text, and Confluence action-item task lists downloaded as nothing (stripped with the ac: cleanup) — a common page element lost in both directions.

Change

Upload: a <ul> where every item starts with a checkbox becomes an ac:task-list with complete/incomplete statuses, preserving inline formatting in the task body; mixed lists stay regular bullets. Download: ac:task-list renders back to - [ ] / - [x] lines with the body's inline formatting. Both directions round-trip.

Compatibility

Existing plain lists are unaffected; only all-checkbox lists change behavior (previously uploaded with literal [ ] text).

Checklist

• ruff check src tests, ruff format --check src tests, python -m pytest tests/ pass locally
• Tests added (both directions, mixed lists, formatting, round trips)
• docs/conversion.md Task lists section + TOC entry
• CHANGELOG.md updated under Unreleased