agent: fix — infinite reactivity loop in TaskSidebar froze entire app

prevTaskIds was declared as $state, causing the $effect that writes to
it to re-trigger itself infinitely (effect_update_depth_exceeded). The
variable is just a comparison reference — doesn't need to be reactive.
Changed to a plain let.

Also removed debug event loggers and reverted grain z-index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/components/TaskSidebar.svelte

@@ -20,7 +20,9 @@
   }
 
   // Track recently added tasks for highlight animation
-  let prevTaskIds = $state(new Set(tasks.map((t) => t.id)));
+  // Use a plain variable (not $state) for previous IDs to avoid
+  // read-write loop inside the $effect.
+  let prevTaskIds = new Set(tasks.map((t) => t.id));
   $effect(() => {
     const currentIds = new Set(tasks.map((t) => t.id));
     for (const id of currentIds) {