重构会话管理功能，由后端 opencode 发放 sessionId，后端做 scope

src/runtime/opencode.ts

@@ -54,14 +54,6 @@ export class OpencodeRuntimeAdapter {
     return requireData(response.data, "session.create");
   }
 
-  async getSession(id: string) {
-    const client = await this.ensureClient();
-    const response = await client.session.get({
-      sessionID: id,
-    });
-    return requireData(response.data, "session.get");
-  }
-
   async sendPrompt(sessionId: string, text: string) {
     await this.prompt(sessionId, text);
     // 当前 SDK 响应风格下，prompt() 本身不会直接返回完整 assistant parts，
@@ -103,15 +95,6 @@ export class OpencodeRuntimeAdapter {
     return requireData(messages.data, "session.messages");
   }
 
-  async forkSession(sessionId: string, messageId?: string) {
-    const client = await this.ensureClient();
-    const response = await client.session.fork({
-      sessionID: sessionId,
-      messageID: messageId,
-    });
-    return requireData(response.data, "session.fork");
-  }
-
   async abortSession(sessionId: string) {
     const client = await this.ensureClient();
     const response = await client.session.abort({