[FEATURE] Session tree: branch history, rollback-and-continue from any message, branch navigation (like Pi)

Feature request

Pi (earendil-works) has tree-shaped session history: you can go back to any earlier point in the conversation, fork a new branch there, and navigate the branches like a git history. OpenCode only has a single-level fork/compact — no way to revisit earlier states of the same session.

What I'd like

In the Desktop app (and TUI):

1. Session timeline / tree view: every fork creates a branch; you can see the whole tree of a session (like a git graph).
2. Rollback to any node: select any earlier message → "continue from here" creates a child branch without losing the current branch (i.e., an explicit fork at any point, not just at continue).
3. Branch navigation: switch between branches of the same session, diff summaries between branches.
4. Visualization: sidebar tab "History/Timeline" in desktop app.

Why

• Current behavior: --fork / fork in desktop clones the whole session; you cannot see where in the conversation the fork happened, and multiple alternative explorations quickly become unmanageable (100+ sessions).
• For debugging AI workflows (try idea A, try idea B from the same point, compare) it's the single most effective pattern.
• Pi has shown the UX; OpenCode has the storage (messages are already stored) — mainly a UI + routing change.

Related

• #34430 (closed: V2 session.fork API) — the API exists; this is the user-facing tree UI.
• #38558 (session.generate contract) — relevant to how branches export.

Proposal

• Treat every session as a DAG/branch tree in storage (parent_id on messages).
• Add /tree slash command or sidebar tab: shows branch nodes, lets you jump/continue.
• --fork becomes "create branch at latest message", new --fork <msg-id> to fork at any message.