Session names should reflect the task being worked on, and the session list should sort newest-first

Summary

Session IDs are random and uninformative (e.g. session_lion_1785967431956_42df39dbb0476edf), so it is very hard to find the session you are looking for in the session picker. A title is only derived from todo state when it exists, so sessions without todos (or early in a session) show as random IDs.

Requested behavior

1. Meaningful session names: each session should get a name that reflects the problem being worked on, not a random ID. The name should be auto-generated from the work itself - for example summarized from the first user prompt / task description, the todo plan intent (user_intention), or the first user message - and shown in the session picker, the header, and the terminal title. The session summary already carries first_user_prompt, title, and custom_title, so the pieces exist to derive a good default label.
2. Newest-first ordering: sessions should be displayed in descending order, newest at the top, so the most recent work is immediately visible when the picker opens.

Environment

• jcode TUI, session picker
• Observed with many sessions: names like session_lion_1785967431956_42df39dbb0476edf make it impossible to tell which session was about which problem without opening each one.

Acceptance criteria

• Every session has a human-readable name derived from its task/problem when a task exists (falling back to the current ID/title behavior otherwise).
• The picker shows the derived name prominently and sorts sessions newest-first by default.
• The header / terminal title uses the same meaningful name.
• Existing manual/custom title behavior (if any) still takes precedence over the derived name.

Related

• #696 asks for manual session renaming and grouping by project folder; this request is about auto-derived names and recency ordering, but the two are complementary.

Start by tracing the session picker and the session summary fields first_user_prompt, title, and custom_title, then follow how the header and terminal title obtain their labels. Implement the derived-name fallback and newest-first ordering while preserving manual or custom titles. Done means the picker, header, and terminal title agree and sessions without task text retain the existing fallback behavior.