feat: implement Trello board management

Add 5 board management tools extending the Trello client:
- trello_create_board — create a new board
- trello_rename_board — rename an existing board
- trello_archive_board — close/archive a board
- trello_open_board — re-open a closed board
- trello_board_details — view board with lists and members

Includes board name-to-ID resolution, 16 new tests, and spec doc.

Issue: #2

docs/backend/manage-boards-spec.md

@@ -0,0 +1,116 @@
+# Trello Plugin — Manage Trello Boards
+
+**Feature:** US: Manage Trello Boards
+**Issue:** #2
+**Branch:** `feature/manage-boards`
+
+## Overview
+
+Extends the Trello plugin with board management capabilities: create, rename, close/archive, open, and view details of Trello boards.
+
+## Design Decisions
+
+- **Extends existing TrelloClient** — All board operations go through the same `TrelloClient` class from the auth feature.
+- **Board selection by ID** — The Trello API identifies boards by ID. The tool accepts both ID and name (client resolves name to ID).
+- **Resolves board name to ID** — When a user provides a board name instead of ID, the client fetches all boards and matches by name.
+
+## Tools
+
+### `trello_create_board`
+Create a new Trello board.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `name` | string | Yes | Board name |
+| `default_lists` | bool | No | Whether to create the default lists (default: true) |
+
+**Returns:**
+```json
+{
+  "success": true,
+  "board": {"id": "abc123", "name": "My Board", "url": "https://trello.com/b/abc123"}
+}
+```
+
+### `trello_rename_board`
+
+Rename an existing board.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `board_id` | string | Yes | Board ID or name |
+| `name` | string | Yes | New board name |
+
+**Returns:**
+```json
+{"success": true, "board": {"id": "abc123", "name": "New Name"}}
+```
+
+### `trello_archive_board`
+
+Close/archive a board.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `board_id` | string | Yes | Board ID or name |
+
+**Returns:**
+```json
+{"success": true, "message": "Board 'My Board' archived."}
+```
+
+### `trello_open_board`
+
+Re-open a closed/archived board.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `board_id` | string | Yes | Board ID or name |
+
+**Returns:**
+```json
+{"success": true, "message": "Board 'My Board' opened."}
+```
+
+### `trello_board_details`
+
+View details of a specific board, including its lists and members.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `board_id` | string | Yes | Board ID or name |
+
+**Returns:**
+```json
+{
+  "success": true,
+  "board": {
+    "id": "abc123",
+    "name": "My Board",
+    "url": "https://trello.com/b/abc123",
+    "desc": "",
+    "closed": false,
+    "starred": false,
+    "lists": [{"id": "l1", "name": "To Do"}, {"id": "l2", "name": "In Progress"}],
+    "members": [{"id": "m1", "username": "user1", "full_name": "User One"}]
+  }
+}
+```
+
+## Trello API Endpoints
+
+| Purpose | Method | Endpoint |
+|---------|--------|----------|
+| Create board | POST | `/1/boards/` |
+| Update board | PUT | `/1/boards/{id}` |
+| Get board details | GET | `/1/boards/{id}` (with lists and members fields) |
+
+## Error Handling
+
+- Board not found → clear message suggesting `trello_list_boards` to find the ID
+- Validation errors → surfaced directly from Trello API