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/client.py

@@ -0,0 +1,175 @@
+"""
+Trello Plugin — Hermes Agent plugin for Trello board integration.
+
+Authentication & Connection module.
+Provides the TrelloClient for interacting with the Trello REST API.
+"""
+
+from __future__ import annotations
+
+import os
+from typing import Any
+
+import requests
+
+TRELLO_API_BASE = "https://api.trello.com/1"
+
+
+class TrelloClient:
+    """Client for the Trello REST API.
+
+    Reads credentials from environment variables TRELLO_API_KEY and TRELLO_TOKEN
+    unless explicitly passed. All public methods return plain dicts suitable for
+    JSON serialization by Hermes tool handlers.
+    """
+
+    def __init__(
+        self,
+        api_key: str | None = None,
+        token: str | None = None,
+        session: requests.Session | None = None,
+    ) -> None:
+        self.api_key = api_key or os.environ.get("TRELLO_API_KEY", "")
+        self.token = token or os.environ.get("TRELLO_TOKEN", "")
+        self._session = session or requests.Session()
+
+    # ------------------------------------------------------------------
+    # Internal helpers
+    # ------------------------------------------------------------------
+
+    def _auth_params(self) -> dict[str, str]:
+        """Return query parameters common to every Trello API call."""
+        return {"key": self.api_key, "token": self.token}
+
+    def _get(self, path: str, params: dict[str, Any] | None = None) -> dict[str, Any]:
+        """Make an authenticated GET request and return the JSON response."""
+        if not self.api_key or not self.token:
+            return {
+                "success": False,
+                "message": "TRELLO_API_KEY and TRELLO_TOKEN must both be set as environment variables.",
+            }
+
+        url = f"{TRELLO_API_BASE}{path}"
+        merged_params = self._auth_params()
+        if params:
+            merged_params.update(params)
+
+        try:
+            resp = self._session.get(url, params=merged_params, timeout=15)
+            resp.raise_for_status()
+            data: dict[str, Any] = resp.json()
+            return {"success": True, "data": data}
+        except requests.exceptions.HTTPError as exc:
+            status = exc.response.status_code if exc.response is not None else 0
+            if status == 401:
+                return {
+                    "success": False,
+                    "message": "Trello authentication failed (401). Check your TRELLO_API_KEY and TRELLO_TOKEN.",
+                }
+            if status == 403:
+                return {
+                    "success": False,
+                    "message": "Trello access denied (403). Your token may not have the required scopes.",
+                }
+            if status == 429:
+                return {
+                    "success": False,
+                    "message": "Trello rate limit exceeded. Try again later.",
+                }
+            return {
+                "success": False,
+                "message": f"Trello API error ({status}): {exc}",
+            }
+        except requests.exceptions.ConnectionError:
+            return {
+                "success": False,
+                "message": "Could not connect to Trello API. Check your network connection.",
+            }
+        except requests.exceptions.Timeout:
+            return {
+                "success": False,
+                "message": "Trello API request timed out. Try again later.",
+            }
+        except requests.exceptions.RequestException as exc:
+            return {
+                "success": False,
+                "message": f"Trello API request failed: {exc}",
+            }
+
+    # ------------------------------------------------------------------
+    # Public API — Auth & Connection
+    # ------------------------------------------------------------------
+
+    def verify_credentials(self) -> dict[str, Any]:
+        """Verify that the stored credentials are valid by fetching member info.
+
+        Returns:
+            dict with ``success`` bool and ``message`` string. On success the
+            ``member`` key contains the Trello member id and username.
+        """
+        result = self._get("/members/me")
+        if result["success"]:
+            member = result["data"]
+            return {
+                "success": True,
+                "message": (
+                    f"✓ Trello credentials verified successfully "
+                    f"(authenticated as {member.get('username', 'unknown')})"
+                ),
+                "member": {
+                    "id": member.get("id"),
+                    "username": member.get("username"),
+                    "full_name": member.get("fullName"),
+                },
+            }
+        return {
+            "success": False,
+            "message": result.get("message", "Trello credential verification failed."),
+        }
+
+    def list_boards(self) -> dict[str, Any]:
+        """Fetch all boards accessible to the authenticated user.
+
+        Returns:
+            dict with ``success`` bool and a ``boards`` list (each containing
+            ``id``, ``name``, ``url``, ``closed``, ``starred``).
+        """
+        result = self._get("/members/me/boards", params={"fields": "id,name,url,closed,starred"})
+        if not result["success"]:
+            return {
+                "success": False,
+                "message": result.get("message", "Failed to list Trello boards."),
+            }
+
+        boards = [
+            {
+                "id": b.get("id"),
+                "name": b.get("name"),
+                "url": b.get("url"),
+                "closed": b.get("closed", False),
+                "starred": b.get("starred", False),
+            }
+            for b in result["data"]
+        ]
+        return {"success": True, "boards": boards}
+
+    def disconnect(self) -> dict[str, Any]:
+        """Clear credentials from the in-memory client.
+
+        Note: this does NOT revoke the Trello token. The user must invalidate
+        it via Trello's account settings if they want full revocation.
+        """
+        self.api_key = ""
+        self.token = ""
+        return {
+            "success": True,
+            "message": (
+                "Trello credentials cleared. "
+                "Set TRELLO_API_KEY and TRELLO_TOKEN again to reconnect."
+            ),
+        }
+
+    @staticmethod
+    def check_requirements() -> bool:
+        """Check if the required environment variables are set."""
+        return bool(os.environ.get("TRELLO_API_KEY")) and bool(os.environ.get("TRELLO_TOKEN"))