fix(electrobun): eliminate $effect/$derived cycles in 3 more components

TaskBoardTab: $effect called loadTasks() which wrote ++pollToken ($state)
→ triggered $effect re-run → infinite loop. Fix: onMount instead.
Also tasksByCol $derived created new objects via .reduce/.filter.

FileBrowser: $effect read openDirs (via new Set(openDirs)) AND wrote to
it (openDirs = s) → infinite loop. Fix: onMount with fresh Set.

CommsTab: $effect called loadChannels()/loadAgents() which wrote $state
→ potential cycle. Fix: onMount instead.

Rule: NEVER use $effect for initialization that writes to $state.
Always use onMount for async init + side effects.

ui-electrobun/src/mainview/CommsTab.svelte

@@ -186,11 +186,12 @@
     }
   }
 
-  $effect(() => {
+  // Use onMount instead of $effect — loadChannels/loadAgents write to $state
+  import { onMount } from 'svelte';
+  onMount(() => {
     loadChannels();
     loadAgents();
     appRpc.addMessageListener('btmsg.newMessage', onNewMessage);
-    // Feature 4: Fallback 30s poll for missed events
     pollTimer = setInterval(() => {
       if (mode === 'channels' && activeChannelId) {
         loadChannelMessages(activeChannelId);
@@ -198,7 +199,6 @@
         loadDmMessages(activeDmAgentId);
       }
     }, 30000);
-
     return () => {
       if (pollTimer) clearInterval(pollTimer);
       appRpc.removeMessageListener?.('btmsg.newMessage', onNewMessage);

ui-electrobun/src/mainview/FileBrowser.svelte

@@ -263,13 +263,13 @@
     return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
   }
 
-  // Load root directory on mount
-  $effect(() => {
+  // Load root directory on mount — use onMount, NOT $effect
+  // $effect reads openDirs (via new Set(openDirs)) and writes to it → infinite loop
+  import { onMount } from 'svelte';
+  onMount(() => {
     if (cwd) {
       loadDir(cwd);
-      const s = new Set(openDirs);
-      s.add(cwd);
-      openDirs = s;
+      openDirs = new Set([cwd]);
     }
   });
 </script>

ui-electrobun/src/mainview/TaskBoardTab.svelte

@@ -51,12 +51,13 @@
 
   // ── Derived ──────────────────────────────────────────────────────────
 
-  let tasksByCol = $derived(
-    COLUMNS.reduce((acc, col) => {
+  // NO $derived — .reduce creates new objects every evaluation → infinite loop
+  function getTasksByCol(): Record<string, Task[]> {
+    return COLUMNS.reduce((acc, col) => {
       acc[col] = tasks.filter(t => t.status === col);
       return acc;
-    }, {} as Record<string, Task[]>)
-  );
+    }, {} as Record<string, Task[]>);
+  }
 
   // ── Data fetching ────────────────────────────────────────────────────
 
@@ -177,10 +178,12 @@
     if (!payload.groupId || payload.groupId === groupId) loadTasks();
   }
 
-  $effect(() => {
+  // Use onMount instead of $effect — loadTasks() writes to $state (pollToken++)
+  // which would trigger $effect re-run → infinite loop
+  import { onMount } from 'svelte';
+  onMount(() => {
     loadTasks();
     appRpc.addMessageListener('bttask.changed', onTaskChanged);
-    // Feature 4: Fallback 30s poll for missed events
     pollTimer = setInterval(loadTasks, 30000);
     return () => {
       if (pollTimer) clearInterval(pollTimer);
@@ -243,11 +246,11 @@
       >
         <div class="tb-col-header">
           <span class="tb-col-label">{COL_LABELS[col]}</span>
-          <span class="tb-col-count">{tasksByCol[col]?.length ?? 0}</span>
+          <span class="tb-col-count">{getTasksByCol()[col]?.length ?? 0}</span>
         </div>
 
         <div class="tb-col-body">
-          {#each tasksByCol[col] ?? [] as task (task.id)}
+          {#each getTasksByCol()[col] ?? [] as task (task.id)}
             <div
               class="tb-card"
               class:dragging={draggedTaskId === task.id}