feat: add tmux-like split panes to boo ui

Generalizes boo ui's single-viewport client-side compositor into a
tiling tree of panes. Each pane is an independent connection to a
session daemon (one socket + one local libghostty terminal), so the
daemon and wire protocol are unchanged.

Bindings (prefix C-a):
- C-a | / C-a -  split the focused pane side-by-side / stacked,
  opening a new session sized to the new pane
- C-a + arrows   move focus between panes when more than one exists
  (otherwise the arrows browse/resize the sidebar as before)
- C-a o          cycle to the next pane
- C-a x          close the focused pane (its session keeps running)

Panes are polled together, rendered with clipped per-pane rows and
dividers, and collapse back into their sibling when a session ends.

Closes #67

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: nkthiebaut

README.md

@@ -81,7 +81,7 @@ Bindings follow GNU screen's defaults, including the `C-x` variants
 | `C-a l`, `C-a C-l` | redraw                     |
 | `C-a a`   | send a literal `C-a`                |
 
-`boo ui` adds additional keybinds for switching, resizing and hiding the sidebar, creating sessions, and killing them.
+`boo ui` adds additional keybinds for switching sessions, resizing and hiding the sidebar, creating and killing sessions, and splitting the view into tmux-like panes (`C-a |` / `C-a -`) navigated with `C-a` and the arrow keys. See `boo help ui` for the full list.
 
 ### Automation
 
@@ -175,8 +175,9 @@ your terminal <-(raw tty)-> boo client <-(unix socket)-> session daemon
 This is a young project, not a drop-in GNU screen replacement:
 
 - One attached client per session (attaching steals); no `-x` sharing.
-- One window per session: no splits or tabs inside a session. Run one
-  session per task and juggle them with `boo ui`.
+- A plain `boo attach` is one window per session: no splits or tabs.
+  `boo ui` tiles sessions into split panes (`C-a |`, `C-a -`), but each
+  pane is still a separate single-window session.
 - The `C-a` prefix is not yet configurable, and pasted bytes containing
   `0x01` are interpreted as the prefix (GNU screen has the same quirk;
   `boo ui` is immune thanks to bracketed paste).

src/help.zig

@@ -138,18 +138,23 @@ pub const commands = [_]Entry{
         \\
         \\keys (prefix C-a, control variants match GNU screen):
         \\  C-a c   create a session and focus it
+        \\  C-a |   split the focused pane left and right, opening a
+        \\          new session in the new pane
+        \\  C-a -   split the focused pane top and bottom, opening a
+        \\          new session in the new pane
+        \\  C-a o   focus the next pane
+        \\  C-a x   close the focused pane (its session keeps running)
         \\  C-a k   kill the focused session (asks y/n)
         \\  C-a r   rename the focused session
         \\  C-a g   go to a session by name (best match)
         \\  C-a n   focus the next session
         \\  C-a p   focus the previous session
-        \\  C-a Up, C-a Down
-        \\          browse the session list without attaching:
-        \\          Up/Down move the selection, Enter attaches it,
-        \\          Esc returns to the focused session
-        \\  C-a Left, C-a Right
-        \\          resize the sidebar: Left/Right adjust the width,
-        \\          Enter keeps it, Esc restores the previous width
+        \\  C-a Up, C-a Down, C-a Left, C-a Right
+        \\          with more than one pane, move focus to the pane in
+        \\          that direction. With a single pane, Up/Down browse
+        \\          the session list (Enter attaches, Esc returns) and
+        \\          Left/Right resize the sidebar (Enter keeps it, Esc
+        \\          restores the previous width)
         \\  C-a s   show or hide the sidebar; the viewport takes the
         \\          full width while it is hidden
         \\  C-a C-a focus the previously focused session