**What was implemented**  
- A lightweight `DeepAgent` base class and a concrete `CustomSearchAgent` that generates deterministic mock search results.  
- The agent creates *virtual files* in memory (`self._virtual_files`) during `search()`.  
- `export_virtual_files()` writes those in‑memory files to a user‑supplied directory.  
- A CLI entry point (`src/run.py`) that runs a search and exports the files.  
- Unit tests (`tests/test_agent.py`) that verify initialization, result generation, virtual‑file creation, and export.

**Why the main parts satisfy the requirements**  
- **Virtual file creation** – `CustomSearchAgent.search()` calls `create_virtual_file()` for each result, storing the content in `self._virtual_files`.  
  ```python
  for idx, (title, snippet) in enumerate(results, start=1):
      filename = f"result_{idx}.txt"
      content = f"Filename: {filename}\nTitle: {title}\nSnippet: {snippet}"
      self.create_virtual_file(filename, content)
  ```
- **Exporting** – `export_virtual_files()` writes every entry in `self._virtual_files` to disk, creating the directory if needed.  
  ```python
  for filename, content in self._virtual_files.items():
      file_path = out_path / filename
      file_path.write_text(content, encoding="utf-8")
  ```
- **No external services** – All data is generated locally; no network calls or APIs are used.  
- **Testability & documentation** – The agent’s public API is simple, and the tests in `tests/test_agent.py` cover all required behaviours.  
- **Executable in the assignment environment** – Running `python -m src.run --query "python" --output "./output"` performs a search and writes the virtual files to `./output`.

**Short code excerpts**  
- `src/agent.py` – base class and virtual‑file handling  
  ```python
  class DeepAgent(ABC):
      def __init__(self) -> None:
          self._virtual_files: Dict[str, str] = {}
  ```
- `src/agent.py` – search logic and file creation  
  ```python
  def search(self, query: str) -> List[Tuple[str, str]]:
      results = self._generate_mock_results(query)
      for idx, (title, snippet) in enumerate(results, start=1):
          filename = f"result_{idx}.txt"
          content = f"Filename: {filename}\nTitle: {title}\nSnippet: {snippet}"
          self.create_virtual_file(filename, content)
      return results
  ```
- `src/run.py` – command‑line integration  
  ```python
  def main() -> None:
      ...
      agent = CustomSearchAgent()
      results = agent.search(args.query)
      ...
      agent.export_virtual_files(output_dir)
  ```

**Honest limitations**  
- The agent does **not** perform real web searches; it returns deterministic mock data, which is sufficient for the assignment but not for production use.  
- File names are limited to simple names without path separators; this is enforced by `create_virtual_file()`.  
- The implementation assumes UTF‑8 encoding for all virtual files.  

Overall, the solution meets all stated constraints: pure Python, no external services, creates and exports virtual files, is testable, and can be run directly from the repository.