refactor: Remove old example scripts and migration references

- Delete old example scripts (mail_reader_leann.py, google_history_reader_leann.py, etc.)
- Remove migration hints and backward compatibility
- Update tests to use new unified examples directly
- Clean up all references to old script names
- Users now only see the new unified interface

tests/test_ci_minimal.py

@@ -20,7 +20,7 @@ def test_package_imports():
 def test_cli_help():
     """Test that CLI example shows help."""
     result = subprocess.run(
-        [sys.executable, "examples/main_cli_example.py", "--help"], capture_output=True, text=True
+        [sys.executable, "examples/document_rag.py", "--help"], capture_output=True, text=True
     )
 
     assert result.returncode == 0