♻️ refactor(vscode): 续聊里的原型段落搬进模板文件

那段正文原先拼在 prompts.ts 里，设置面板看不到也改不了。现在落到
prompts/brainstorm-continue-prototype.md，面板「提示词」组多一项「续聊·原型段落」，
占位符 {prototypeFile} / {issueNumber} / {prototypeSessionId}；取值为空的整行会被
删掉，免得留下半句没有值的说明。

同时把措辞收紧：原型改动只能 ListAgents + SendMessage 给已开着的原型会话，
点名禁止派子代理 / fork 分身 / 自己动插件代码。

Claude-Session: https://claude.ai/code/session_01Kgmb1LvrPmFX3NoUDB4zZC

vscode/src/panel/handlers/settings.ts

@@ -24,6 +24,7 @@ export async function handleSettingsSave(panel: KanbanWebviewPanel, payload: {
   webhookSecret: string
   brainstormPrompt: string
   brainstormContinuePrompt: string
+  prototypeRefPrompt: string
   implementPlanPrompt: string
   autoReview: boolean
   reviewPrompt: string
@@ -114,6 +115,7 @@ export async function handleSettingsSave(panel: KanbanWebviewPanel, payload: {
       webhookSecret: payload.webhookSecret,
       brainstormPrompt: payload.brainstormPrompt || prev.brainstormPrompt,
       brainstormContinuePrompt: payload.brainstormContinuePrompt || prev.brainstormContinuePrompt,
+      prototypeRefPrompt: payload.prototypeRefPrompt || prev.prototypeRefPrompt,
       implementPlanPrompt: payload.implementPlanPrompt || prev.implementPlanPrompt,
       autoReview: payload.autoReview,
       reviewPrompt: payload.reviewPrompt || prev.reviewPrompt,
@@ -149,6 +151,7 @@ export async function handleSettingsSave(panel: KanbanWebviewPanel, payload: {
     webhookSecret: payload.webhookSecret.trim(),
     brainstormPrompt: payload.brainstormPrompt,
     brainstormContinuePrompt: payload.brainstormContinuePrompt,
+    prototypeRefPrompt: payload.prototypeRefPrompt,
     implementPlanPrompt: payload.implementPlanPrompt,
     autoReview: payload.autoReview,
     reviewPrompt: payload.reviewPrompt,
@@ -261,6 +264,7 @@ export async function handleEditSettingsRequest(panel: KanbanWebviewPanel): Prom
     webhookSecret: s.webhookSecret,
     brainstormPrompt: s.brainstormPrompt,
     brainstormContinuePrompt: s.brainstormContinuePrompt,
+    prototypeRefPrompt: s.prototypeRefPrompt,
     implementPlanPrompt: s.implementPlanPrompt,
     autoReview: s.autoReview,
     reviewPrompt: s.reviewPrompt,