feat: implement Trello authentication & connection plugin

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

.gitignore

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+__pycache__/
+*.egg-info/
+*.pyc
+*.pyo
+.pytest_cache/