docs: session_name string + liveness cache + background ticker

docs: remove s keybinding from spec — Enter is the sole session action

docs: remove s keybinding — Enter handles both link and switch

docs: update plan — Enter opens picker when no session, remove LinkedAt

docs: update spec and plan for single session, cwd pre-fill, 15-row detail, session picker

docs: add implementation plan

docs: opportunistic session cleanup, multiline description

docs: revise spec based on review feedback

docs: add hive TUI design spec