fix(a2a): remove stale acp __pycache__ directory left from ACP→A2A rename

Add BDD tests to verify that cleveragents.acp module is not importable and
the acp directory does not exist in the source tree. This ensures v3.6.0
deliverable #1 compliance: 'No acp references in public API; all imports
use a2a namespace'.

The acp module was renamed to a2a in issue #688, but the __pycache__
directory was left behind, allowing the module to still be imported from
bytecode. This test verifies the fix is complete.

features/a2a_acp_module_removed.feature

@@ -0,0 +1,13 @@
+Feature: ACP module is not importable
+  As a CleverAgents developer
+  I want to ensure the old ACP module is completely removed
+  So that the v3.6.0 deliverable #1 requirement is met: "No acp references in public API"
+
+  Scenario: Importing cleveragents.acp raises ImportError
+    When I attempt to import the cleveragents.acp module
+    Then an ImportError should be raised
+    And the error message should indicate the module does not exist
+
+  Scenario: The acp directory does not exist in the source tree
+    When I check the source tree for the acp directory
+    Then the src/cleveragents/acp directory should not exist

features/steps/a2a_acp_module_removed_steps.py

@@ -0,0 +1,74 @@
+"""Step definitions for a2a_acp_module_removed.feature.
+
+Tests that the old ACP module is completely removed and not importable,
+ensuring v3.6.0 deliverable #1 compliance: "No acp references in public API".
+"""
+
+from __future__ import annotations
+
+import importlib
+import sys
+from pathlib import Path
+from typing import Any
+
+from behave import then, when
+
+
+@when("I attempt to import the cleveragents.acp module")
+def step_attempt_import_acp(context: Any) -> None:
+    """Attempt to import cleveragents.acp and capture any ImportError."""
+    context.import_error = None
+    try:
+        # Remove from sys.modules if it exists to ensure fresh import
+        if "cleveragents.acp" in sys.modules:
+            del sys.modules["cleveragents.acp"]
+
+        # Attempt to import
+        importlib.import_module("cleveragents.acp")
+        context.import_succeeded = True
+    except ImportError as e:
+        context.import_error = e
+        context.import_succeeded = False
+
+
+@then("an ImportError should be raised")
+def step_check_import_error(context: Any) -> None:
+    """Verify that an ImportError was raised."""
+    assert context.import_error is not None, (
+        "Expected ImportError to be raised when importing cleveragents.acp, "
+        "but import succeeded"
+    )
+    assert not context.import_succeeded, (
+        "Expected import to fail, but it succeeded"
+    )
+
+
+@then("the error message should indicate the module does not exist")
+def step_check_error_message(context: Any) -> None:
+    """Verify that the error message indicates the module doesn't exist."""
+    error_msg = str(context.import_error)
+    assert "cleveragents.acp" in error_msg or "No module named" in error_msg, (
+        f"Expected error message to mention 'cleveragents.acp' or 'No module named', "
+        f"but got: {error_msg}"
+    )
+
+
+@when("I check the source tree for the acp directory")
+def step_check_acp_directory(context: Any) -> None:
+    """Check if the acp directory exists in the source tree."""
+    # Get the path to the cleveragents package
+    import cleveragents
+
+    cleveragents_path = Path(cleveragents.__file__).parent
+    acp_path = cleveragents_path / "acp"
+
+    context.acp_directory_exists = acp_path.exists()
+    context.acp_path = acp_path
+
+
+@then("the src/cleveragents/acp directory should not exist")
+def step_check_acp_not_exists(context: Any) -> None:
+    """Verify that the acp directory does not exist."""
+    assert not context.acp_directory_exists, (
+        f"Expected acp directory to not exist, but found it at: {context.acp_path}"
+    )