Add TrelloClient for Trello REST API interaction with env-var-based credential management (TRELLO_API_KEY, TRELLO_TOKEN). Implements: - trello_verify_credentials — verify API key/token against Trello API - trello_list_boards — list accessible Trello boards - trello_disconnect — clear in-memory credentials Includes full test suite (18 tests), spec document, and plugin metadata. Issue: #1