feat(tui): hide and show the tail with t

docs/design.md

@@ -3116,6 +3116,14 @@ read. A cropped pane is read whole whatever the band's height, and the rows
 above its cut fill the band from the bottom, so a taller band shows more of
 what the agent last said.
 
+`t` hides the band and gives its rows to the forest, and `t` again brings it
+back. While the band is hidden `bdi` reads no pane, since nothing would be
+drawn from the read: a read already out is answered into nothing, and none is
+due. Bringing the band back reads the selected pane at once, and until that
+answer lands the band says it is reading rather than show rows as old as the
+hiding. The band is shown at every start. The choice lasts the run and is not
+remembered.
+
 ### The reader says what their background is
 
 `bdi` cannot see the reader's background and does not ask. A `[theme]` section
@@ -3473,6 +3481,7 @@ The bindings are vim-like, with the arrows as aliases:
 | `s` | cycle which copy of a bead opens, under the selected node |
 | `S` | cycle which copy of a bead opens, across the whole forest |
 | `F` | draw the selected bead as the only root, or put the forest back |
+| `t` | hide or show the pane's output under the forest |
 | `/` | find part of a bead's id or title, wherever the forest draws it |
 | `n`, `^G` | go to the next bead matching the search |
 | `N`, `^T` | go to the one before it |

src/tui/drive.rs

@@ -629,6 +629,7 @@ fn answered(
                 | Action::CycleSpine
                 | Action::CycleSpineForest
                 | Action::ToggleFilter
+                | Action::ToggleTail
                 | Action::FocusForest
                 | Action::Search
                 | Action::NextMatch

src/tui/keys.rs

@@ -109,6 +109,12 @@ pub(super) const BINDINGS: &[Binding] = &[
         does: "draw the selected bead as the only root, or put the forest back",
         hint: None,
     },
+    Binding {
+        keys: &[alone(KeyCode::Char('t'), "t")],
+        action: Action::ToggleTail,
+        does: "hide or show the pane's output under the forest",
+        hint: None,
+    },
     Binding {
         keys: &[alone(KeyCode::Char('/'), "/")],
         action: Action::Search,
@@ -538,6 +544,7 @@ pub(super) mod tests {
             Action::CycleSpine,
             Action::CycleSpineForest,
             Action::ToggleFilter,
+            Action::ToggleTail,
             Action::Focus,
             Action::FocusForest,
             Action::ShowBead,
@@ -574,6 +581,7 @@ pub(super) mod tests {
                 | Action::CycleSpine
                 | Action::CycleSpineForest
                 | Action::ToggleFilter
+                | Action::ToggleTail
                 | Action::Focus
                 | Action::FocusForest
                 | Action::ShowBead