feat(KB-129): dashboard performance optimizations

- Fix SSE hook cleanup to prevent memory leaks and stale connections
- Cap agent log memory and optimize batch log processing
- Memoize Board, Column, and TaskCard with custom comparator to reduce re-renders
- Stabilize column task arrays and preserve pagination across live updates
- Add TaskCardBadge component for PR/issue state display
- Remove deprecated GitHub polling code and archive functionality from core store

packages/dashboard/app/components/Board.tsx

@@ -1,8 +1,8 @@
-import type { Task, TaskDetail, TaskCreateInput, Column as ColumnType } from "@kb/core";
+import type { Task, TaskDetail, Column as ColumnType } from "@kb/core";
 import { COLUMNS } from "@kb/core";
 import { Column } from "./Column";
 import type { ToastType } from "../hooks/useToast";
-import { useState, useMemo } from "react";
+import { useState, useMemo, useCallback, useRef } from "react";
 
 interface BoardProps {
   tasks: Task[];
@@ -24,8 +24,36 @@ interface BoardProps {
   searchQuery?: string;
 }
 
+function sortTasksForColumn(tasks: Task[]): Task[] {
+  return [...tasks].sort((a, b) => {
+    if (a.columnMovedAt && b.columnMovedAt) {
+      return b.columnMovedAt.localeCompare(a.columnMovedAt);
+    }
+    if (a.columnMovedAt && !b.columnMovedAt) return -1;
+    if (!a.columnMovedAt && b.columnMovedAt) return 1;
+    return a.createdAt.localeCompare(b.createdAt);
+  });
+}
+
+function areTaskArraysEqual(previous: Task[], next: Task[]): boolean {
+  if (previous.length !== next.length) return false;
+  return previous.every((task, index) => task === next[index]);
+}
+
 export function Board({ tasks, maxConcurrent, onMoveTask, onOpenDetail, addToast, onQuickCreate, onNewTask, autoMerge, onToggleAutoMerge, globalPaused, onUpdateTask, onArchiveTask, onUnarchiveTask, searchQuery = "" }: BoardProps) {
   const [archivedCollapsed, setArchivedCollapsed] = useState(true);
+  const tasksByColumnCacheRef = useRef<Record<ColumnType, Task[]>>({
+    triage: [],
+    todo: [],
+    "in-progress": [],
+    "in-review": [],
+    done: [],
+    archived: [],
+  });
+
+  const handleToggleArchivedCollapse = useCallback(() => {
+    setArchivedCollapsed((current) => !current);
+  }, []);
 
   // Filter tasks based on search query (matches id, title, or description)
   const filteredTasks = useMemo(() => {
@@ -39,25 +67,38 @@ export function Board({ tasks, maxConcurrent, onMoveTask, onOpenDetail, addToast
     );
   }, [tasks, searchQuery]);
 
+  // Keep per-column array identities stable for unchanged columns so React.memo(Column)
+  // can skip sibling rerenders during unrelated task updates.
+  const tasksByColumn = useMemo(() => {
+    const nextGrouped = Object.fromEntries(
+      COLUMNS.map((column) => [column, [] as Task[]]),
+    ) as Record<ColumnType, Task[]>;
+
+    for (const task of filteredTasks) {
+      nextGrouped[task.column].push(task);
+    }
+
+    const previousGrouped = tasksByColumnCacheRef.current;
+    const stableGrouped = {} as Record<ColumnType, Task[]>;
+
+    for (const column of COLUMNS) {
+      const sortedTasks = sortTasksForColumn(nextGrouped[column]);
+      stableGrouped[column] = areTaskArraysEqual(previousGrouped[column], sortedTasks)
+        ? previousGrouped[column]
+        : sortedTasks;
+    }
+
+    tasksByColumnCacheRef.current = stableGrouped;
+    return stableGrouped;
+  }, [filteredTasks]);
+
   return (
     <main className="board" id="board">
       {COLUMNS.map((col) => (
         <Column
           key={col}
           column={col}
-          tasks={filteredTasks
-            .filter((t) => t.column === col)
-            .sort((a, b) => {
-              // Tasks with columnMovedAt sort descending (most recent first)
-              // Tasks without it (legacy) fall to the bottom, sorted by createdAt ascending
-              if (a.columnMovedAt && b.columnMovedAt) {
-                return b.columnMovedAt.localeCompare(a.columnMovedAt);
-              }
-              if (a.columnMovedAt && !b.columnMovedAt) return -1;
-              if (!a.columnMovedAt && b.columnMovedAt) return 1;
-              return a.createdAt.localeCompare(b.createdAt);
-            })}
-          allTasks={tasks}
+          tasks={tasksByColumn[col]}
           maxConcurrent={maxConcurrent}
           onMoveTask={onMoveTask}
           onOpenDetail={onOpenDetail}
@@ -68,7 +109,7 @@ export function Board({ tasks, maxConcurrent, onMoveTask, onOpenDetail, addToast
           onUnarchiveTask={onUnarchiveTask}
           {...(col === "triage" ? { onQuickCreate, onNewTask } : {})}
           {...(col === "in-review" ? { autoMerge, onToggleAutoMerge } : {})}
-          {...(col === "archived" ? { collapsed: archivedCollapsed, onToggleCollapse: () => setArchivedCollapsed(!archivedCollapsed) } : {})}
+          {...(col === "archived" ? { collapsed: archivedCollapsed, onToggleCollapse: handleToggleArchivedCollapse } : {})}
         />
       ))}
     </main>