Anchored edits are now atomic

The edit_content tool used to check its text anchor against a snapshot read before the write, so an edit landing in between could be silently overwritten — and a write that never applied could still report "Edited". The check-and-replace now runs against the live document inside the owning hub's write turn: two people (or agents) editing different parts of the same document at once both keep their changes, an anchor that was edited away is refused with a clear message, and the success message is only reported once the edit has provably landed.