chore: 清理 src 下 113 项未使用导入和死代码

删除未使用的文件（BuiltinStatusLine.tsx、4 个重复的 .ts stub）、
移除约 55 个文件中未使用的 React 导入、
清理约 50 处未使用的导入/变量/参数。
净减少 ~296 行代码，precheck 4077 测试全部通过。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/messages/SnipBoundaryMessage.ts

@@ -1,4 +0,0 @@
-// Auto-generated stub — replace with real implementation
-export {}
-export const SnipBoundaryMessage: (props: Record<string, unknown>) => null =
-  () => null

src/components/messages/UserCrossSessionMessage.ts

@@ -1,4 +0,0 @@
-// Auto-generated stub — replace with real implementation
-export {}
-export const UserCrossSessionMessage: (props: Record<string, unknown>) => null =
-  () => null

src/components/messages/UserForkBoilerplateMessage.ts

@@ -1,5 +0,0 @@
-// Auto-generated stub — replace with real implementation
-export {}
-export const UserForkBoilerplateMessage: (
-  props: Record<string, unknown>,
-) => null = () => null

src/components/messages/UserGitHubWebhookMessage.ts

@@ -1,5 +0,0 @@
-// Auto-generated stub — replace with real implementation
-export {}
-export const UserGitHubWebhookMessage: (
-  props: Record<string, unknown>,
-) => null = () => null