feat: 支持添加默认提示词（兼容块）

- prompt-profiles.js: 新增 createLegacyPromptBlock 导出
- panel.js: 新增 add-legacy-block 处理逻辑
- 工具栏新增 '+ 默认提示词' 按钮

panel.js

@@ -7,6 +7,7 @@ import {
   cloneTaskProfile,
   createBuiltinPromptBlock,
   createCustomPromptBlock,
+  createLegacyPromptBlock,
   createLocalRegexRule,
   ensureTaskProfiles,
   exportTaskProfile as serializeTaskProfile,
@@ -1680,6 +1681,15 @@ async function _handleTaskProfileWorkspaceClick(event) {
         return { selectBlockId: nextBlock.id };
       });
       return;
+    case "add-legacy-block":
+      _updateCurrentTaskProfile((draft, context) => {
+        const nextBlock = createLegacyPromptBlock(context.taskType, {
+          order: draft.blocks.length,
+        });
+        draft.blocks.push(nextBlock);
+        return { selectBlockId: nextBlock.id };
+      });
+      return;
     case "add-builtin-block": {
       const select = document.getElementById("bme-task-builtin-select");
       const sourceKey = String(select?.value || "").trim();
@@ -1947,6 +1957,9 @@ function _renderTaskPromptTab(state) {
             <button class="bme-config-secondary-btn" data-task-action="add-custom-block" type="button">
               + 自定义块
             </button>
+            <button class="bme-config-secondary-btn" data-task-action="add-legacy-block" type="button">
+              + 默认提示词
+            </button>
             <span class="bme-task-action-sep"></span>
             <select id="bme-task-builtin-select" class="bme-config-input bme-task-builtin-select">
               ${state.builtinBlockDefinitions

prompt-profiles.js

@@ -363,6 +363,23 @@ export function createBuiltinPromptBlock(taskType, sourceKey = "", overrides = {
   });
 }
 
+export function createLegacyPromptBlock(taskType, overrides = {}) {
+  const legacyField = LEGACY_PROMPT_FIELD_MAP[taskType] || "";
+  return normalizePromptBlock(taskType, {
+    id: createPromptBlockId(taskType),
+    name: "默认提示词",
+    type: "legacyPrompt",
+    enabled: true,
+    role: "system",
+    sourceKey: "",
+    sourceField: legacyField,
+    content: "",
+    injectionMode: "append",
+    order: 0,
+    ...overrides,
+  });
+}
+
 export function createLocalRegexRule(taskType, overrides = {}) {
   return normalizeRegexLocalRule(
     {