fix(core): remove per-prompt system option (#34361)

packages/core/test/session-runner.test.ts

@@ -894,36 +894,7 @@ describe("SessionRunnerLLM", () => {
     }),
   )
 
-  it.effect("appends the per-request prompt system after the agent prompt and durable baseline", () =>
-    Effect.gen(function* () {
-      yield* setup
-      const agent = yield* AgentV2.Service
-      yield* agent.transform((editor) =>
-        editor.update(AgentV2.ID.make("build"), (agent) => {
-          agent.system = "Build agent instructions"
-          agent.mode = "primary"
-        }),
-      )
-      const session = yield* SessionV2.Service
-      yield* session.prompt({
-        sessionID,
-        prompt: Prompt.make({ text: "First", system: "Per-request override" }),
-        resume: false,
-      })
-
-      requests.length = 0
-      response = fragmentFixture("text", "text-system", ["Done"]).completeEvents
-      yield* session.resume(sessionID)
-
-      expect(requests.at(-1)?.system.map((part) => part.text)).toEqual([
-        "Build agent instructions",
-        "Initial context",
-        "Per-request override",
-      ])
-    }),
-  )
-
-  it.effect("omits the per-request system part when the prompt has no system string", () =>
+  it.effect("uses only the agent prompt and durable baseline as system parts", () =>
     Effect.gen(function* () {
       yield* setup
       const agent = yield* AgentV2.Service