feat: 添加多代理系统和 Task 子任务功能

Agent 模块:
- types.ts: Agent 类型定义 (AgentInfo, AgentPermission 等)
- permission-merger.ts: 三级权限合并逻辑 (系统->全局->Agent)
- config-loader.ts: 支持 YAML/JSON 配置文件加载
- registry.ts: Agent 注册表，管理预设和自定义 Agent
- executor.ts: Agent 执行器，支持工具过滤和权限控制
- presets/: 预设 Agent (general, explore, code-reviewer, build, plan)

Task 工具:
- task.ts: 执行子任务，委派给指定 Agent 处理
- 支持子会话创建和管理

会话扩展:
- 支持父子会话关系 (parentId, agentName)
- 新增 createChildSession, saveChildSession 方法

配置:
- 支持 .ai-assist/agents.yaml 用户自定义 Agent
- 支持通配符模式的 Bash 权限规则

package-lock.json

@@ -16,6 +16,7 @@
         "chalk": "^5.3.0",
         "commander": "^12.1.0",
         "inquirer": "^12.0.0",
+        "js-yaml": "^4.1.1",
         "ora": "^8.1.0",
         "tree-sitter-bash": "^0.25.1",
         "vscode-jsonrpc": "^8.2.1",
@@ -27,6 +28,7 @@
         "ai-assist": "dist/index.js"
       },
       "devDependencies": {
+        "@types/js-yaml": "^4.0.9",
         "@types/node": "^22.0.0",
         "tsx": "^4.19.0",
         "typescript": "^5.6.0"
@@ -912,6 +914,13 @@
         "js-tiktoken": "^1.0.14"
       }
     },
+    "node_modules/@types/js-yaml": {
+      "version": "4.0.9",
+      "resolved": "https://registry.npmjs.org/@types/js-yaml/-/js-yaml-4.0.9.tgz",
+      "integrity": "sha512-k4MGaQl5TGo/iipqb2UDG2UwjXziSWkh0uysQelTlJpX1qGlpUZYm8PnO4DxG1qBomtJUdYJ6qR6xdIah10JLg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/node": {
       "version": "22.19.2",
       "resolved": "https://registry.npmjs.org/@types/node/-/node-22.19.2.tgz",
@@ -985,6 +994,12 @@
         "url": "https://github.com/chalk/ansi-styles?sponsor=1"
       }
     },
+    "node_modules/argparse": {
+      "version": "2.0.1",
+      "resolved": "https://registry.npmjs.org/argparse/-/argparse-2.0.1.tgz",
+      "integrity": "sha512-8+9WqebbFzpX9OR+Wa6O29asIogeRMzcGtAINdpMHHyAg10f05aSFVBbcEqGf/PXw1EjAZ+q2/bEBg3DvurK3Q==",
+      "license": "Python-2.0"
+    },
     "node_modules/asynckit": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/asynckit/-/asynckit-0.4.0.tgz",
@@ -1540,6 +1555,18 @@
         "base64-js": "^1.5.1"
       }
     },
+    "node_modules/js-yaml": {
+      "version": "4.1.1",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.1.1.tgz",
+      "integrity": "sha512-qQKT4zQxXl8lLwBtHMWwaTcGfFOZviOJet3Oy/xmGk2gZH677CJM9EvtfdSkgWcATZhj/55JZ0rmy3myCT5lsA==",
+      "license": "MIT",
+      "dependencies": {
+        "argparse": "^2.0.1"
+      },
+      "bin": {
+        "js-yaml": "bin/js-yaml.js"
+      }
+    },
     "node_modules/json-schema": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/json-schema/-/json-schema-0.4.0.tgz",