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

src/trello_plugin/__init__.py

@@ -0,0 +1,25 @@
+"""
+trello-plugin — Hermes Agent plugin for Trello board integration.
+"""
+
+from trello_plugin.tools import (
+    PLUGIN_DESCRIPTION,
+    PLUGIN_NAME,
+    PLUGIN_TOOLS,
+    PLUGIN_VERSION,
+    check_requirements,
+    trello_disconnect,
+    trello_list_boards,
+    trello_verify_credentials,
+)
+
+__all__ = [
+    "PLUGIN_NAME",
+    "PLUGIN_DESCRIPTION",
+    "PLUGIN_VERSION",
+    "PLUGIN_TOOLS",
+    "check_requirements",
+    "trello_verify_credentials",
+    "trello_list_boards",
+    "trello_disconnect",
+]