Ctrl+\ puts the current file beside the others, in a second editor group

[a-team-app]

Ctrl+\ (or mg, or View → Move to other group) moves the active tab into a second editor group on the right, so a test can sit beside the code it covers. Each group has its own tab strip and they split the width evenly. F6 (or fo) moves the keys to the other group, and so does a click. jg puts every tab back in one group. A group whose last tab leaves or closes goes away, and the other one takes the full width.

Closes #460

A file is open in one group at most: opening it from the Explorer, Ctrl+O, a Find result, Back/Forward or the Review tab focuses it where it already is. Commands on the active tab act on the focused group. Save all, quit, rename/move/delete, disk-change notices and Find/Replace in files cover both groups. The split, and which group had focus, come back when the folder is reopened.

Choices worth a look:

• With no split, Focus other group and Join groups don't run, so the palette and leader don't offer them and the menu dims them. Move to other group runs whenever a tab is open, so mg on a lone tab can say Open another file to split with.
• The left group keeps the Editor title on its border. The right group's border has no title.
• F1 lists Ctrl+\ and F6 in the Editor, Diff and Tabs columns.
• Opening a diff that's already open in the other group opens a second copy in the focused group. The Review tab is the exception: it finds the diff in either group. I left the rest alone because diffs are out of scope here.

🤖 Generated with Claude Code

a-team try tuicode 500

[a-team-app]

Windows CI at 449cb36 failed with two ViewedFilesHostTests hitting RunAsync timed out. They passed on Windows at f2ea711 on this branch, and the full suite passes locally, so I've re-run the job.

Heads-up: Windows RunAsync timed out has now hit three different host tests on this branch (SidebarDrag, then FocusHostTests, then these two), and none on main or other branches in the last 25 failed runs. If it keeps happening, the branch is likely making Windows host tests flaky (the new focusable editor frames or the per-iteration focus reconcile are the suspects), not just the runner being slow.