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/tools.py

@@ -0,0 +1,96 @@
+"""
+Trello Plugin — Hermes Agent tool definitions.
+
+Each function decorated with ``@tool`` registers itself as a Hermes tool
+that the agent can invoke during a conversation.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+from typing import Any
+
+from trello_plugin.client import TrelloClient
+
+# ---------------------------------------------------------------------------
+# Module-level client (lazily initialised so env vars can be overridden)
+# ---------------------------------------------------------------------------
+
+_client: TrelloClient | None = None
+
+
+def _get_client() -> TrelloClient:
+    global _client  # noqa: PLW0603
+    api_key = os.environ.get("TRELLO_API_KEY", "")
+    token = os.environ.get("TRELLO_TOKEN", "")
+    if _client is None or _client.api_key != api_key or _client.token != token:
+        _client = TrelloClient(api_key=api_key, token=token)
+    return _client
+
+
+# ---------------------------------------------------------------------------
+# Tool helpers
+# ---------------------------------------------------------------------------
+
+def _respond(data: dict[str, Any]) -> str:
+    """Wrap a result dict in a JSON string, the Hermes tool contract."""
+    return json.dumps(data, indent=2)
+
+
+# ---------------------------------------------------------------------------
+# Tools
+# ---------------------------------------------------------------------------
+
+def trello_verify_credentials() -> str:
+    """Verify that the Trello API key and token are valid.
+
+    Checks the ``TRELLO_API_KEY`` and ``TRELLO_TOKEN`` environment variables
+    and attempts a test call to the Trello API. Returns the authenticated
+    user's Trello username on success.
+    """
+    client = _get_client()
+    result = client.verify_credentials()
+    return _respond(result)
+
+
+def trello_list_boards() -> str:
+    """List all Trello boards accessible to the authenticated user.
+
+    Requires ``TRELLO_API_KEY`` and ``TRELLO_TOKEN`` to be set.
+    Returns board id, name, URL, and whether the board is closed/starred.
+    """
+    client = _get_client()
+    result = client.list_boards()
+    return _respond(result)
+
+
+def trello_disconnect() -> str:
+    """Disconnect from Trello by clearing credentials from memory.
+
+    This does NOT revoke the Trello token — invalidate it via
+    Trello's account settings if full revocation is desired.
+    """
+    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."})
+
+
+# ---------------------------------------------------------------------------
+# Plugin metadata (used by Hermes plugin loader)
+# ---------------------------------------------------------------------------
+
+PLUGIN_NAME = "trello-plugin"
+PLUGIN_DESCRIPTION = "Hermes Agent plugin for Trello board integration"
+PLUGIN_VERSION = "0.1.0"
+PLUGIN_TOOLS = [
+    trello_verify_credentials,
+    trello_list_boards,
+    trello_disconnect,
+]
+PLUGIN_REQUIRES_ENV = ["TRELLO_API_KEY", "TRELLO_TOKEN"]
+
+
+def check_requirements() -> bool:
+    """Return True when both Trello env vars are present."""
+    return TrelloClient.check_requirements()