feat(KB-651): add changed files diff viewer for tasks

- Add backend API endpoint to get changed files and diffs for a task worktree
- Add useChangedFiles hook for fetching and managing diff data
- Create ChangedFilesModal component with file list and diff viewer
- Integrate modal into TaskCard with click handler to view changes
- Add frontend API integration and type definitions
- Include unit tests for hook and modal components
- Add changeset for the new feature
- Remove deprecated mission-related test files

packages/dashboard/app/hooks/useChangedFiles.ts

@@ -0,0 +1,66 @@
+import { useEffect, useState } from "react";
+import { fetchTaskFileDiffs, type TaskFileDiff } from "../api";
+
+const ACTIVE_COLUMNS = new Set(["in-progress", "in-review"]);
+
+interface UseChangedFilesResult {
+  files: TaskFileDiff[];
+  loading: boolean;
+  error: string | null;
+  selectedFile: TaskFileDiff | null;
+  setSelectedFile: (file: TaskFileDiff) => void;
+}
+
+export function useChangedFiles(taskId: string, worktree: string | undefined, column: string): UseChangedFilesResult {
+  const [files, setFiles] = useState<TaskFileDiff[]>([]);
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [selectedFile, setSelectedFile] = useState<TaskFileDiff | null>(null);
+
+  useEffect(() => {
+    if (!taskId || !worktree || !ACTIVE_COLUMNS.has(column)) {
+      setFiles([]);
+      setLoading(false);
+      setError(null);
+      setSelectedFile(null);
+      return;
+    }
+
+    let cancelled = false;
+
+    async function load() {
+      setLoading(true);
+      setError(null);
+      try {
+        const result = await fetchTaskFileDiffs(taskId);
+        if (cancelled) return;
+        setFiles(result);
+        setSelectedFile((current) => {
+          if (result.length === 0) return null;
+          if (current) {
+            const match = result.find((file) => file.path === current.path && file.oldPath === current.oldPath);
+            if (match) return match;
+          }
+          return result[0] ?? null;
+        });
+      } catch (err) {
+        if (cancelled) return;
+        setFiles([]);
+        setSelectedFile(null);
+        setError(err instanceof Error ? err.message : "Failed to load changed files");
+      } finally {
+        if (!cancelled) {
+          setLoading(false);
+        }
+      }
+    }
+
+    void load();
+
+    return () => {
+      cancelled = true;
+    };
+  }, [taskId, worktree, column]);
+
+  return { files, loading, error, selectedFile, setSelectedFile };
+}