[idea] Move tabs between spaces by dragging to the sidebar

[DebGit96]

Right now tabs stay in the workspace where they were created. You can drag tabs horizontally to reorder them in the tab bar, but there is no way to move an existing tab over to another space.

When a long-running shell job or agent task ends up belonging in a different workspace or worktree, the only workaround is opening a new tab in the other space and restarting the work from scratch.

Suggested behavior

Dragging a tab off the tab bar and dropping it onto a space row in the sidebar moves the tab into that space:

1. When dragging a tab, moving the mouse over the sidebar highlights the hovered space as the drop target.
2. Releasing the mouse transfers the tab and its pane layout into that space, keeping running processes, terminal scrollback, and agent status intact.

Details to consider

• Cross-machine dragging: dragging should stay within the same endpoint (local to local, or remote machine to the same remote machine).
• Last tab in a space: if someone drags the only tab out of a workspace, the source workspace could either close (similar to how pane moves handle empty workspaces) or the drag could be refused to avoid accidental workspace closure.

[DebGit96]

Just to add: I've looked through the layout and client drag state in the codebase, and would be very happy to help put together an implementation / draft PR for this if the maintainers are aligned on the direction.