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/useProjectFileEditor.ts

@@ -1,6 +1,6 @@
 import { useState, useEffect, useCallback } from "react";
 import type { FileContentResponse, SaveFileResponse } from "../api";
-import { fetchProjectFileContent, saveProjectFileContent } from "../api";
+import { fetchWorkspaceFileContent, saveWorkspaceFileContent } from "../api";
 
 interface UseProjectFileEditorReturn {
   content: string;
@@ -56,7 +56,7 @@ export function useProjectFileEditor(
       setError(null);
 
       try {
-        const response: FileContentResponse = await fetchProjectFileContent(filePath!);
+        const response: FileContentResponse = await fetchWorkspaceFileContent("project", filePath!);
 
         if (!cancelled) {
           setContentState(response.content);
@@ -95,7 +95,7 @@ export function useProjectFileEditor(
     setError(null);
 
     try {
-      const response: SaveFileResponse = await saveProjectFileContent(filePath, content);
+      const response: SaveFileResponse = await saveWorkspaceFileContent("project", filePath, content);
       setOriginalContent(content);
       setMtime(response.mtime);
     } catch (err: any) {