Add a keyboard shortcut (ArrowUp on empty input) to start editing the
most recent user message, mirroring Mux's behavior. The shortcut reuses
the existing history-edit flow triggered by the pencil button.
Extract a shared `getEditableUserMessagePayload` helper so the pencil
button and the new shortcut both derive the edit payload identically.
Derive the last editable user message during render in
`AgentDetailInput` from the existing store selectors, keeping the
implementation Effect-free and React Compiler friendly.
Ethan
83b2f85d63