[idea] Optional tree-style tabs in the sidebar under workspaces and worktrees

[Troskiev83]

Problem

When working with multiple Git worktrees, the Spaces sidebar provides a clean hierarchical tree: the primary workspace as the parent, with worktree checkouts neatly grouped and indented beneath it.

However, the tabs belonging to those workspaces live exclusively in a disconnected horizontal bar at the top or bottom (tab_bar_position). If you run a multi-worktree workflow where each worktree checkout has dedicated tabs (e.g. agent, logs, server), the mental model is fragmented: you select the workspace in the left sidebar tree, but then have to look over to the horizontal bar to see and switch tabs.

Proposed idea

An optional configuration (for example under [ui] or [ui.sidebar.spaces], such as show_tabs = true or a vertical tabs mode) to render tabs directly as expandable child nodes under each workspace and worktree in the Spaces panel.

Why it matters

• Unified mental hierarchy: Everything from Workspace -> Worktree -> Tab would be visible in one continuous vertical tree.
• Faster context switching: You could see at a glance which tabs exist across parallel worktrees without needing to focus each workspace first just to inspect its horizontal tab bar.
• Consistent navigation: Opening, closing, and selecting tabs could be done directly from the sidebar tree alongside workspaces.