refactor(session): make SystemPrompt a proper Effect Service (#21992)

packages/opencode/test/session/snapshot-tool-race.test.ts

@@ -41,6 +41,7 @@ import { Plugin } from "../../src/plugin"
 import { Provider as ProviderSvc } from "../../src/provider/provider"
 import { Question } from "../../src/question"
 import { Skill } from "../../src/skill"
+import { SystemPrompt } from "../../src/session/system"
 import { Todo } from "../../src/session/todo"
 import { SessionCompaction } from "../../src/session/compaction"
 import { Instruction } from "../../src/session/instruction"
@@ -157,6 +158,7 @@ function makeHttp() {
       Layer.provideMerge(registry),
       Layer.provideMerge(trunc),
       Layer.provide(Instruction.defaultLayer),
+      Layer.provide(SystemPrompt.defaultLayer),
       Layer.provideMerge(deps),
     ),
   )