feat(KB-175): move file browser to header with workspace support

- Add workspace file APIs and hooks for session-scoped file browsing
- Create WorkspaceSelector component for switching between workspaces
- Update FileBrowserModal with workspace mode and improved UX
- Add files button to header with workspace-aware file browsing
- Add session files indicator to task cards for quick file access
- Include comprehensive tests for all new components and hooks

packages/dashboard/app/hooks/useWorkspaceFileBrowser.ts

@@ -0,0 +1,92 @@
+import { useState, useEffect, useCallback } from "react";
+import type { FileNode, FileListResponse } from "../api";
+import { fetchWorkspaceFileList } from "../api";
+
+interface UseWorkspaceFileBrowserReturn {
+  entries: FileNode[];
+  currentPath: string;
+  setPath: (path: string) => void;
+  loading: boolean;
+  error: string | null;
+  refresh: () => void;
+}
+
+/**
+ * Hook for browsing files in a selected workspace.
+ *
+ * @param workspace - The workspace identifier ("project" or task ID)
+ * @param enabled - Whether fetching is enabled
+ */
+export function useWorkspaceFileBrowser(
+  workspace: string,
+  enabled: boolean,
+): UseWorkspaceFileBrowserReturn {
+  const [entries, setEntries] = useState<FileNode[]>([]);
+  const [currentPath, setCurrentPath] = useState<string>(".");
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [refreshKey, setRefreshKey] = useState(0);
+
+  const refresh = useCallback(() => {
+    setRefreshKey((key) => key + 1);
+  }, []);
+
+  const setPath = useCallback((path: string) => {
+    setCurrentPath(path);
+    setError(null);
+  }, []);
+
+  useEffect(() => {
+    setCurrentPath(".");
+    setError(null);
+    setEntries([]);
+  }, [workspace]);
+
+  useEffect(() => {
+    if (!enabled || !workspace) {
+      return;
+    }
+
+    let cancelled = false;
+
+    async function loadFiles() {
+      setLoading(true);
+      setError(null);
+
+      try {
+        const response: FileListResponse = await fetchWorkspaceFileList(
+          workspace,
+          currentPath === "." ? undefined : currentPath,
+        );
+
+        if (!cancelled) {
+          setEntries(response.entries);
+        }
+      } catch (err: any) {
+        if (!cancelled) {
+          setError(err.message || "Failed to load files");
+          setEntries([]);
+        }
+      } finally {
+        if (!cancelled) {
+          setLoading(false);
+        }
+      }
+    }
+
+    void loadFiles();
+
+    return () => {
+      cancelled = true;
+    };
+  }, [workspace, currentPath, enabled, refreshKey]);
+
+  return {
+    entries,
+    currentPath,
+    setPath,
+    loading,
+    error,
+    refresh,
+  };
+}