fix: address PR #5 review comments

- Add class-level type annotations on TrelloClient (api_key, token, _session)
- Add TypedDict response contracts with TypeGuard narrowing for _request
- Replace os.environ manipulation with monkeypatch in tests
- Align disconnect message with auto-reconnect behavior
- Add docstring for TRELLO_API_BASE constant
- Add _post and _put request helpers for future endpoints

src/trello_plugin/tools.py

@@ -73,7 +73,7 @@ def trello_disconnect() -> str:
     """
     global _client  # noqa: PLW0603
     _client = None
-    return _respond({"success": True, "message": "Trello credentials cleared. Reconnect by calling trello_verify_credentials after setting TRELLO_API_KEY and TRELLO_TOKEN."})
+    return _respond({"success": True, "message": "Trello credentials cleared from memory. Next tool call will re-read TRELLO_API_KEY and TRELLO_TOKEN from environment and reconnect automatically."})
 
 
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