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>

tddai/config.py

@@ -46,16 +46,20 @@ class TddaiConfig:
         config = cls()
 
         # Override with environment variables if present
-        if gitea_url := os.getenv("TDDAI_GITEA_URL"):
+        gitea_url = os.getenv("TDDAI_GITEA_URL")
+        if gitea_url:
             config.gitea_url = gitea_url
 
-        if repo_owner := os.getenv("TDDAI_REPO_OWNER"):
+        repo_owner = os.getenv("TDDAI_REPO_OWNER")
+        if repo_owner:
             config.repo_owner = repo_owner
 
-        if repo_name := os.getenv("TDDAI_REPO_NAME"):
+        repo_name = os.getenv("TDDAI_REPO_NAME")
+        if repo_name:
             config.repo_name = repo_name
 
-        if workspace_dir := os.getenv("TDDAI_WORKSPACE_DIR"):
+        workspace_dir = os.getenv("TDDAI_WORKSPACE_DIR")
+        if workspace_dir:
             config.workspace_dir = Path(workspace_dir)
 
         return config