refactor: simplify unified diff to always use persistent ClaudeCodeBuffer in current tab

- The unified diff no longer auto-opens a window or creates tab 2.
-  prepares  silently.
- Added  and  for user-initiated
  buffer access (works in any tab, creates a vsplit if needed).
- Cleanup closes any window showing  and keeps the
  buffer alive showing the reviewed file.
- Removed , , tab2 logic from
  this path; those remain for native diff.

This eliminates the tab-3 confusion: the diff always lives in
 and the user decides where to view it.

Author: spoloxs