[BUG] TUI: horizontal separator line renders truncated in narrow terminal windows

In a narrow terminal window, the horizontal separator line rendered below the local-command-caveat notice block (and around the "(Branch)" indicator) doesn't span the full width of the pane — it renders as a short teal line stopping partway across, unlike other horizontal rules in the UI which span the full width.

Steps to reproduce:

1. Run /branch to fork the current session
2. Narrow the terminal window
3. Observe the separator line rendered below the local-command-caveat / branch notice block

Expected: the separator spans the full pane width, consistent with other horizontal rules.
Actual: the separator is truncated partway across the screen.

Screenshot to be added as a follow-up comment.