fix(workbench): Keep tool window resizing smooth over a terminal or long conversation (#137)

Dragging the bottom tool window edge over a terminal refit xterm on every
pointer move and sent each intermediate size to the shell, which redrew its
prompt every time. Terminals now refit at most every 100 ms while their host
keeps resizing, plus once when it stops, and the process only learns the size
once it has held for 150 ms. This covers the Terminal tool window and terminals
embedded in Run consoles.

Resizing the width of an agent conversation reflowed every entry, including the
off-screen ones (14-16 ms per move in a long session). Entries now use
content-visibility: auto, so only the visible ones are laid out.

src/features/agents/Session.module.css

@@ -117,6 +117,11 @@
 .stream:focus {
   outline: none;
 }
+/* Off-screen entries skip layout, so resizing the pane doesn't reflow a long conversation. */
+.stream > * {
+  content-visibility: auto;
+  contain-intrinsic-block-size: auto 80px;
+}
 
 /* Your message with its actions, shown on hover. */
 .message {