fix: Python 3.6 compatibility for tests

- Replace walrus operator (:=) with traditional assignment in config.py
- Replace datetime.fromisoformat() with strptime() for Python 3.6
- Replace subprocess capture_output and text params with PIPE and universal_newlines
- All tests now pass on Python 3.6.9

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_issue_11_workflow_integration.py

@@ -9,6 +9,7 @@ This test validates issue #11: Setup TDD workspace infrastructure
 
 import pytest
 import subprocess
+from subprocess import PIPE
 import tempfile
 import shutil
 from pathlib import Path
@@ -32,7 +33,7 @@ class TestWorkflowIntegration:
     def test_make_tdd_status_command(self):
         """Test that make tdd-status command works correctly."""
         result = subprocess.run(['make', 'tdd-status'],
-                              capture_output=True, text=True)
+                              stdout=PIPE, stderr=PIPE, universal_newlines=True)
 
         assert result.returncode == 0, "tdd-status command should succeed"
         # Should show clean workspace when no active workspace
@@ -42,7 +43,7 @@ class TestWorkflowIntegration:
     def test_make_tdd_add_test_command_without_workspace(self):
         """Test that make tdd-add-test provides proper error when no workspace."""
         result = subprocess.run(['make', 'tdd-add-test'],
-                              capture_output=True, text=True)
+                              stdout=PIPE, stderr=PIPE, universal_newlines=True)
 
         assert result.returncode != 0, "tdd-add-test command should fail when no workspace"
         assert "No active issue workspace" in result.stdout