fix: retain submitted questions and answers in conversations

Preserve read-only question and answer reviews after successful submission or skipping, including multiple rounds and local conversation reloads. Keep failed submissions retryable and preserve newer questions and terminal states when responses arrive late.

Validation: typecheck and lint passed; full suite had 3900 passes, 66 skips and one Agent package timeout that passed on isolated rerun. Real OpenCode UI acceptance was not run. Previously discarded answers cannot be recovered.

Release note: 修复 OpenCode 提交回答后会话仅显示已提交的问题；现在保留原问题及对应答案，多轮问答和跳过记录在重新打开会话后仍可查看。

src/main/assistant/assistant-database.ts

@@ -275,6 +275,7 @@ type MessageMetadata = {
   imageContextNotice?: ConversationSnapshot['messages'][number]['imageContextNotice']
   task?: ConversationSnapshot['messages'][number]['task']
   attachments?: ConversationSnapshot['messages'][number]['attachments']
+  answeredQuestions?: ConversationSnapshot['messages'][number]['answeredQuestions']
 }
 
 const MAX_CHANNEL_OUTBOX_RETRY_BYTES = 20 * 1024 * 1024
@@ -1163,7 +1164,8 @@ function toConversationSnapshot(
         artifactIds: metadata.artifactIds,
         imageContextNotice: metadata.imageContextNotice,
         task: metadata.task,
-        attachments: metadata.attachments
+        attachments: metadata.attachments,
+        answeredQuestions: metadata.answeredQuestions
       }
     })
   }
@@ -1189,7 +1191,8 @@ function serializeConversationMessageMetadata(
     artifactIds: message.artifactIds,
     imageContextNotice: message.imageContextNotice,
     task: message.task,
-    attachments: message.attachments
+    attachments: message.attachments,
+    answeredQuestions: message.answeredQuestions
   })
 }
 
@@ -4633,7 +4636,8 @@ export class AssistantDatabase {
         knowledgeRetrieval: metadata.knowledgeRetrieval,
         artifactIds: metadata.artifactIds,
         task: metadata.task,
-        attachments: metadata.attachments
+        attachments: metadata.attachments,
+        answeredQuestions: metadata.answeredQuestions
       })
       const failed = reduceRecoveredAgentEvent(current, {
         requestId: taskId,
@@ -5041,7 +5045,8 @@ export class AssistantDatabase {
         knowledgeRetrieval: metadata.knowledgeRetrieval,
         artifactIds: metadata.artifactIds,
         task: metadata.task,
-        attachments: metadata.attachments
+        attachments: metadata.attachments,
+        answeredQuestions: metadata.answeredQuestions
       })
       const reduced = reduceRecoveredAgentEvent(message, publicEvent)
       const messageUpdate = database