fix: 聊天产出文件行拦截改用 slots.inject 延迟注册（issue #15）

conversation.chat.turnTail 是宿主 ui-conversation 在 conversation.chat.node
children 表中声明的子插槽，直接 slots.register 会早于声明而抛
"not declared (a parent entry's children table must declare it)"，
导致拦截注册失败回退默认 deliverables 行为并弹诊断条。

改用 ctx.slots.inject 等待声明落账后再注册（与官方
dsh-client-ui-deliverables 同款写法）：已声明则同步注册，未声明则
在声明提交时注册，声明撤销自动卸载、重声明重新注册，fiber 卸载
幂等清理。editor tab 禁用门控与 openInSidebar 座位不变。

新增 tests/turn-tail-intercept.spec.ts 覆盖已声明/待声明/提前销毁
三种注册时机、editor 禁用回退与 openInSidebar 座位接线。

src/client/intercept.tsx

@@ -57,9 +57,22 @@ export function SidebarProducedFiles(props: {
   )
 }
 
-/** Register the turn-tail interception (returns the disposer). */
+/**
+ * Register the turn-tail interception (returns the disposer).
+ *
+ * The slot is a CHILD slot the host's ui-conversation declares in its
+ * `conversation.chat.node` children table (kind: chain, scope: session).
+ * Registering it directly races the declaration — the ui-slots core's
+ * load-time validation throws "not declared (a parent entry's children
+ * table must declare it)" when the parent entry is not on the ledger yet.
+ * slots.inject waits for the declaration: the callback runs synchronously
+ * when the slot is already declared, otherwise it runs inside the declaring
+ * register() call once the declaration commits; declaration collapse
+ * disposes the entry and a later declaration re-registers it. This mirrors
+ * @deepseek-ai/dsh-client-ui-deliverables' registration of the same slot.
+ */
 export function registerTurnTailInterception(ctx: Context, store: SidebarStore): () => void {
-  return ctx.slots.register({
+  return ctx.slots.inject('conversation.chat.turnTail', () => ctx.slots.register({
     name: 'conversation.chat.turnTail',
     // Decline the takeover while the editor tab type is disabled in the side
     // card settings: the produced-files row falls back to the default
@@ -73,7 +86,7 @@ export function registerTurnTailInterception(ctx: Context, store: SidebarStore):
     inject: (sessionId: string) => ({
       openInSidebar: (path: string) => { openSidebarFile(ctx, store, sessionId, path) },
     }),
-  }, SidebarProducedFiles)
+  }, SidebarProducedFiles))
 }
 
 /**