fix: previous conversation history never injected on resume (#314)

Check session existence in ChatService before passing previousConversation
to the agent. Only pass it for new sessions — existing sessions already
have real conversation history in the GeminiClient.

apps/server/src/agent/gemini-agent.ts

@@ -397,8 +397,7 @@ ${message}
 
     // Inject previous conversation if resuming (no server-side history)
     let fullMessage = userQuery
-    const hasHistory = this.client.getHistory().length > 0
-    if (previousConversation && !hasHistory) {
+    if (previousConversation) {
       fullMessage = `<previous_conversation>
 The user is resuming a previous conversation. Here is the conversation history for context:
 

apps/server/src/api/services/chat-service.ts

@@ -99,13 +99,14 @@ export class ChatService {
       isScheduledTask: request.isScheduledTask,
     }
 
+    const isNewSession = !sessionManager.has(request.conversationId)
     const agent = await sessionManager.getOrCreate(agentConfig, mcpServers)
     await agent.execute(
       request.message,
       rawStream,
       abortSignal,
       request.browserContext,
-      request.previousConversation,
+      isNewSession ? request.previousConversation : undefined,
     )
   }