Investigate syncing with browsers?

Is there value in letting you push your local bookmarks into your browser's bookmark list? How about pulling your browser's bookmarks into `book`?

Things to think about:
- How do we handle non HTTP(S) bookmarks if we push to the browser?
- How do we handle the extra metadata from browsers that just aren't relevant to us in book?
- How do we handle conflicts between the two?
- How do we handle syncing from browser, and then syncing to the browser, without a loss of browser-specific metadata?

The issue names no files, tests, or entry points. Start by surveying how `book` represents bookmarks and where its CLI handles import or export, then compare those boundaries with browser bookmark formats. Define supported synchronization directions, metadata preservation, non-HTTP(S) handling, and conflict behavior before implementation; done means the scope and acceptance criteria are agreed.

Written by the indexing model from the issue text.