feat: implement expand-task MCP command

- Create direct function wrapper in expand-task.js with error handling

- Add MCP tool integration for breaking down tasks into subtasks

- Update task-master-core.js to expose expandTaskDirect function

- Update changeset to document the new command

- Parameter support for subtask generation options (num, research, prompt, force)

mcp-server/src/core/direct-functions/expand-task.js

@@ -0,0 +1,152 @@
+/**
+ * expand-task.js
+ * Direct function implementation for expanding a task into subtasks
+ */
+
+import { expandTask } from '../../../../scripts/modules/task-manager.js';
+import { readJSON, writeJSON } from '../../../../scripts/modules/utils.js';
+import { findTasksJsonPath } from '../utils/path-utils.js';
+import path from 'path';
+import fs from 'fs';
+
+/**
+ * Direct function wrapper for expanding a task into subtasks with error handling.
+ *
+ * @param {Object} args - Command arguments
+ * @param {Object} log - Logger object
+ * @returns {Promise<Object>} - Task expansion result { success: boolean, data?: any, error?: { code: string, message: string }, fromCache: boolean }
+ */
+export async function expandTaskDirect(args, log) {
+  let tasksPath;
+  try {
+    // Find the tasks path first
+    tasksPath = findTasksJsonPath(args, log);
+  } catch (error) {
+    log.error(`Tasks file not found: ${error.message}`);
+    return { 
+      success: false, 
+      error: { 
+        code: 'FILE_NOT_FOUND_ERROR', 
+        message: error.message 
+      }, 
+      fromCache: false 
+    };
+  }
+
+  // Validate task ID
+  const taskId = args.id ? parseInt(args.id, 10) : null;
+  if (!taskId) {
+    log.error('Task ID is required');
+    return { 
+      success: false, 
+      error: { 
+        code: 'INPUT_VALIDATION_ERROR', 
+        message: 'Task ID is required' 
+      }, 
+      fromCache: false 
+    };
+  }
+
+  // Process other parameters
+  const numSubtasks = args.num ? parseInt(args.num, 10) : undefined;
+  const useResearch = args.research === true;
+  const additionalContext = args.prompt || '';
+  const force = args.force === true;
+
+  try {
+    log.info(`Expanding task ${taskId} into ${numSubtasks || 'default'} subtasks. Research: ${useResearch}, Force: ${force}`);
+    
+    // Read tasks data
+    const data = readJSON(tasksPath);
+    if (!data || !data.tasks) {
+      return { 
+        success: false, 
+        error: { 
+          code: 'INVALID_TASKS_FILE', 
+          message: `No valid tasks found in ${tasksPath}` 
+        }, 
+        fromCache: false
+      };
+    }
+    
+    // Find the specific task
+    const task = data.tasks.find(t => t.id === taskId);
+    
+    if (!task) {
+      return { 
+        success: false, 
+        error: { 
+          code: 'TASK_NOT_FOUND', 
+          message: `Task with ID ${taskId} not found` 
+        }, 
+        fromCache: false
+      };
+    }
+    
+    // Check if task is completed
+    if (task.status === 'done' || task.status === 'completed') {
+      return { 
+        success: false, 
+        error: { 
+          code: 'TASK_COMPLETED', 
+          message: `Task ${taskId} is already marked as ${task.status} and cannot be expanded` 
+        }, 
+        fromCache: false
+      };
+    }
+    
+    // Check for existing subtasks
+    const hasExistingSubtasks = task.subtasks && task.subtasks.length > 0;
+    
+    // Keep a copy of the task before modification
+    const originalTask = JSON.parse(JSON.stringify(task));
+    
+    // Tracking subtasks count before expansion
+    const subtasksCountBefore = task.subtasks ? task.subtasks.length : 0;
+    
+    // Create a backup of the tasks.json file
+    const backupPath = path.join(path.dirname(tasksPath), 'tasks.json.bak');
+    fs.copyFileSync(tasksPath, backupPath);
+    
+    // Directly modify the data instead of calling the CLI function
+    if (!task.subtasks) {
+      task.subtasks = [];
+    }
+    
+    // Save tasks.json with potentially empty subtasks array
+    writeJSON(tasksPath, data);
+    
+    // Call the core expandTask function
+    await expandTask(taskId, numSubtasks, useResearch, additionalContext);
+    
+    // Read the updated data
+    const updatedData = readJSON(tasksPath);
+    const updatedTask = updatedData.tasks.find(t => t.id === taskId);
+    
+    // Calculate how many subtasks were added
+    const subtasksAdded = updatedTask.subtasks ? 
+      updatedTask.subtasks.length - subtasksCountBefore : 0;
+    
+    // Return the result
+    log.info(`Successfully expanded task ${taskId} with ${subtasksAdded} new subtasks`);
+    return { 
+      success: true, 
+      data: { 
+        task: updatedTask,
+        subtasksAdded,
+        hasExistingSubtasks
+      }, 
+      fromCache: false 
+    };
+  } catch (error) {
+    log.error(`Error expanding task: ${error.message}`);
+    return { 
+      success: false, 
+      error: { 
+        code: 'CORE_FUNCTION_ERROR', 
+        message: error.message || 'Failed to expand task' 
+      }, 
+      fromCache: false 
+    };
+  }
+}