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