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/cli/src/http/routes/web/session-index.ts

@@ -11,6 +11,15 @@ export function registerSessionIndexRoutes(server: FastifyInstance, adapter: Ses
     return { sessionCount }
   })
 
+  server.post<{
+    Params: { id: string }
+    Body: { gapThreshold?: number }
+  }>('/_web/sessions/:id/generate-incremental-index', async (request) => {
+    const gapThreshold = (request.body as any)?.gapThreshold ?? 1800
+    const sessionCount = sessionIndexService.generateIncrementalIndex(adapter, request.params.id, gapThreshold)
+    return { sessionCount }
+  })
+
   server.post<{ Params: { id: string } }>('/_web/sessions/:id/clear-index', async (request) => {
     sessionIndexService.clearIndex(adapter, request.params.id)
     return { success: true }