perf(agents): switch back to a long conversation without rebuilding it (#1710)

Agent task tabs now stay mounted while another tab is active (the five most
recently used per pane). A hidden one skips rendering with
content-visibility: hidden, which keeps its layout and scroll offset, so
showing it again costs a few milliseconds instead of building and laying out
the whole conversation. Hidden tabs don't mark their task read and close Find.

Thinking blocks and tool output now sit in a wrapper that skips off-screen
layout like other entries; on <details> itself WebKit wouldn't expand them,
so they were laid out in full on every resize and reopen.

src/features/agents/Session.module.css

@@ -157,7 +157,10 @@
 .stream:focus {
   outline: none;
 }
-/* Skip off-screen output layout; editable messages must repaint immediately when focus/content changes. */
+/*
+ * Skip off-screen output layout; editable messages must repaint immediately when focus/content changes.
+ * Thinking and tool output sit in a <div>: on <details> itself WebKit wouldn't expand them.
+ */
 .stream > :not(.message):not(details) {
   content-visibility: auto;
   contain-intrinsic-block-size: auto 80px;