…ready

- add a backend tool-terminal-ready event with tool_use_id and terminal_session_id
- emit the event when bash terminal sessions are created for both foreground and background runs
- bind terminalSessionId onto flow chat tool items with out-of-order event handling
- update TerminalToolCard to switch from loading to open-terminal once the session is ready
- refine tool card header divider for single-status-icon states