feat(sync): use incremental session index after pull sync

After pull-sync imports new messages, generate session index incrementally
instead of full regeneration. This preserves existing sessions and their
summaries, only processing newly imported (unindexed) messages.

Adds generateIncremental method across the full stack:
- core: already had generateIncrementalSessionIndex
- node-runtime: expose generateIncrementalIndex
- CLI: add POST /_web/sessions/:id/generate-incremental-index route
- Electron: add session:generateIncremental IPC handler
- Frontend: add generateIncremental to SessionIndexAdapter interface

apps/desktop/main/ipc/chat.ts

@@ -803,6 +803,15 @@ export function registerChatHandlers(ctx: IpcContext): void {
     }
   })
 
+  ipcMain.handle('session:generateIncremental', async (_, sessionId: string, gapThreshold?: number) => {
+    try {
+      return await worker.generateIncrementalSessions(sessionId, gapThreshold)
+    } catch (error) {
+      console.error('Failed to generate incremental session index:', error)
+      throw error
+    }
+  })
+
   /**
    * 检查是否已生成会话索引
    */

apps/desktop/preload/apis/utils.ts

@@ -240,9 +240,10 @@ export const sessionApi = {
     return ipcRenderer.invoke('session:generate', sessionId, gapThreshold)
   },
 
-  /**
-   * 检查是否已生成会话索引
-   */
+  generateIncremental: (sessionId: string, gapThreshold?: number): Promise<number> => {
+    return ipcRenderer.invoke('session:generateIncremental', sessionId, gapThreshold)
+  },
+
   hasIndex: (sessionId: string): Promise<boolean> => {
     return ipcRenderer.invoke('session:hasIndex', sessionId)
   },

apps/desktop/preload/index.d.ts

@@ -1107,6 +1107,7 @@ interface ChatSessionItem {
 
 interface SessionApi {
   generate: (sessionId: string, gapThreshold?: number) => Promise<number>
+  generateIncremental: (sessionId: string, gapThreshold?: number) => Promise<number>
   hasIndex: (sessionId: string) => Promise<boolean>
   getStats: (sessionId: string) => Promise<SessionStats>
   clear: (sessionId: string) => Promise<boolean>