feat(desktop): rest the thinking caret at a faint hint instead of invisible

A run of thinking headers is the one place the disclosure affordance isn't
otherwise discoverable — every other one sits in a row you're already
reaching for. The resting opacity becomes a token so only that surface opts
in; DisclosureRow is shared with tool rows and run headers, which keep the
invisible-until-hover default.

apps/desktop/src/components/chat/disclosure-row.tsx

@@ -55,7 +55,7 @@ export function DisclosureRow({
               'flex h-(--conversation-line-height) shrink-0 items-center justify-center transition-opacity duration-150',
               open
                 ? 'opacity-80'
-                : 'opacity-0 group-hover/disclosure-row:opacity-80 group-focus-within/disclosure-row:opacity-80'
+                : 'opacity-(--disclosure-caret-rest) group-hover/disclosure-row:opacity-80 group-focus-within/disclosure-row:opacity-80'
             )}
           >
             <DisclosureCaret open={open} />

apps/desktop/src/styles.css

@@ -413,6 +413,9 @@
     /* Tight gap between tool rows inside a single action group, so a back-to-back
        run still reads as one cohesive sequence. */
     --tool-row-gap: 0.375rem;
+    /* Resting opacity of a disclosure caret — invisible until hover, except
+       where a surface opts into a faint hint of the affordance. */
+    --disclosure-caret-rest: 0;
     /* Paragraph spacing — vertical gap between prose paragraphs, both inside a
        markdown block and between consecutive prose parts. Single knob; tweak
        freely. */
@@ -1548,6 +1551,14 @@ text-* variant utilities. */ .btn-arc {
   margin-block: var(--turn-block-gap);
 }
 
+/* Thinking headers carry a faint resting caret. A run of them is the one place
+   the affordance isn't otherwise discoverable — every other disclosure sits in
+   a row you're already reaching for. The scaffold fade multiplies this, so the
+   painted value lands well under the number here. */
+[data-slot='aui_thinking-disclosure'] {
+  --disclosure-caret-rest: 0.4;
+}
+
 /* A streaming turn is sealed into several assistant bubbles as it goes
    (`message.interim`) and rehydrates into fewer, so the same two blocks are
    sometimes siblings inside one bubble and sometimes split across two. The