Add conversation branching to insight chat (rewind preserves forks)

Store chat history as a tree in training_messages instead of a flat
array. Rewind now sets active_leaf_id to the target node rather than
truncating, so discarded paths survive as alternate branches. Fork
indicators ("X/Y") mark divergence points and let the client load or
switch to alternate paths.

- llm_client: StoredChatNode/ChatHistoryStore with tree traversal
  helpers (path_to_leaf, children_of, fork_at_node, leaves*, etc.) and
  from_flat_array for backward-compatible reads of the old flat format.
- insight_chat: load_history/chat_turn/rewind_history operate on the
  tree; new switch_branch and get_branches. render_tree_path walks every
  node (including non-rendered tool-dispatch nodes) when computing fork
  info, so a fork whose diverging child is a tool call still surfaces on
  the following rendered message.
- handlers: GET /insights/chat/branches, POST /insights/chat/switch-branch,
  and a branch_id param on the history endpoint.

Backward-compatible: flat arrays are converted to a tree lazily on read.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -377,6 +377,8 @@ fn main() -> std::io::Result<()> {
                 .service(ai::chat_stream_handler)
                 .service(ai::chat_history_handler)
                 .service(ai::chat_rewind_handler)
+                .service(ai::chat_branches_handler)
+                .service(ai::chat_switch_branch_handler)
                 .service(ai::turn_async_handler)
                 .service(ai::turn_replay_handler)
                 .service(ai::cancel_turn_handler)