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/useSessionFiles.ts

@@ -0,0 +1,46 @@
+import { useEffect, useState } from "react";
+import { fetchSessionFiles } from "../api";
+
+const ACTIVE_COLUMNS = new Set(["in-progress", "in-review"]);
+
+interface UseSessionFilesResult {
+  files: string[];
+  loading: boolean;
+}
+
+export function useSessionFiles(taskId: string, worktree: string | undefined, column: string): UseSessionFilesResult {
+  const [files, setFiles] = useState<string[]>([]);
+  const [loading, setLoading] = useState(false);
+
+  useEffect(() => {
+    if (!taskId || !worktree || !ACTIVE_COLUMNS.has(column)) {
+      setFiles([]);
+      setLoading(false);
+      return;
+    }
+
+    let cancelled = false;
+
+    async function load() {
+      setLoading(true);
+      try {
+        const result = await fetchSessionFiles(taskId);
+        if (!cancelled) {
+          setFiles(result);
+        }
+      } catch {
+        if (!cancelled) {
+          setFiles([]);
+        }
+      } finally {
+        if (!cancelled) {
+          setLoading(false);
+        }
+      }
+    }
+
+    void load();
+  }, [taskId, worktree, column]);
+
+  return { files, loading };
+}