Feature: tree-view of the current session traversal (like Pi's tree feature)

Feature request
Add a tree/exploration view of the current session traversal, similar to the "tree" feature in the Pi coding agent, that lets users see and navigate the branch structure of the current session.

What it should do

• Render the current session as a tree: the root prompt, branch points, forks, and each parallel/derived thread (e.g. subagents, swarm children, forked panes).
• Let users traverse the tree to jump to a specific point/thread rather than scrolling a linear transcript.
• Make visible where the active position is in the tree.

Why it benefits all users

• Long sessions with forks, subagents, and swarm orchestration quickly become hard to navigate linearly. A tree gives a mental map of how the work is branching and where the current focus is.
• Encourages a consistent way to inspect and jump across the session graph that today is only partially covered by pane/tab switching and /fork.

Suggested scope
A read-only tree view in the side panel keyed off the existing session/turn graph (forked panes, subagent children, swarm nodes are already tracked), with the ability to select a node and open/jump to that point.

Start by tracing the existing session/turn graph and the pane/tab, /fork, subagent, and swarm navigation paths. Confirm how a read-only side-panel view can identify the root, forks, child threads, and active position. Done means users can inspect the tree and select a node to jump to its corresponding point or thread.