feat(ai-chat): dim stale TodoWrite blocks and neutralise their icons

Adds .ai-msg-tool.ai-todo-stale modifier that lowers a TodoWrite
block's opacity to 0.65 and overrides .ai-todo-icon colour to a
neutral muted tone with reduced alpha. Paired with the browser-side
sweep that swaps the stale spinner glyph for fa-arrow-right and
strips the in_progress class, so older TodoWrite cards read as
historical snapshots rather than implying parallel activity.

Author: abose

src/styles/Extn-AIChatPanel.less

@@ -794,6 +794,20 @@
     &.pending { color: @project-panel-text-2; opacity: 0.4; }
 }
 
+// Older TodoWrite blocks are dimmed and have their in-progress spinner
+// swapped for a static arrow when a newer TodoWrite supersedes them.
+// Only the latest block keeps live animation; everything above reads
+// as historical "this was the next step at the time" rather than
+// implying parallel activity.
+.ai-msg-tool.ai-todo-stale {
+    opacity: 0.65;
+
+    .ai-todo-icon {
+        color: @project-panel-text-2;
+        opacity: 0.6;
+    }
+}
+
 .ai-todo-content {
     color: @project-panel-text-2;
     line-height: 1.4;