fix(opencode): add prompt caching to session compaction - Issue #10342

- Added SystemPrompt import to compaction.ts
- Changed system from empty array to include environment and custom prompts
- Enables ~90% cost savings on cached system prompts during compaction
- Aligns compaction behavior with normal request pattern
- Resolves performance and cost issue where system prompts were sent repeatedly without caching

Benefits:
- Reduced API costs (cached tokens cost ~90% less)
- Faster compaction responses
- Consistent behavior between normal requests and compaction

Technical details:
- System prompts now included in cache control strategy
- Matches pattern used in prompt.ts for normal requests

Related: Issue #10342

Author: root

packages/opencode/src/session/compaction.ts

@@ -7,6 +7,7 @@ import { Provider } from "../provider/provider"
 import { MessageV2 } from "./message-v2"
 import z from "zod"
 import { SessionPrompt } from "./prompt"
+import { SystemPrompt } from "./system"
 import { Token } from "../util/token"
 import { Log } from "../util/log"
 import { SessionProcessor } from "./processor"
@@ -147,7 +148,7 @@ export namespace SessionCompaction {
       abort: input.abort,
       sessionID: input.sessionID,
       tools: {},
-      system: [],
+      system: [...(await SystemPrompt.environment()), ...(await SystemPrompt.custom())],
       messages: [
         ...MessageV2.toModelMessages(input.messages, model),
         {