core: fix missing Flag import in session prompt

packages/opencode/src/session/prompt.ts

@@ -5,6 +5,7 @@ import z from "zod"
 import { Identifier } from "../id/id"
 import { MessageV2 } from "./message-v2"
 import { Log } from "../util/log"
+import { Flag } from "../flag/flag"
 import { SessionRevert } from "./revert"
 import { Session } from "."
 import { Agent } from "../agent/agent"