perf(compaction): reuse message update parts for no-text tails

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/preemptive-compaction.degradation-monitor.test.ts

@@ -192,4 +192,28 @@ describe("preemptive-compaction post-compaction degradation monitor", () => {
     // then
     expect(ctx.client.session.summarize).not.toHaveBeenCalled()
   })
+
+  it("uses message update parts without refetching session messages", async () => {
+    // given
+    const sessionHistory: AssistantHistoryMessage[] = []
+    const ctx = createMockCtx(sessionHistory)
+    const hook = createPreemptiveCompactionHook(ctx as never, {} as never)
+    const sessionID = "ses_tail_update_parts"
+    const stepOnlyParts = [{ type: "step-start" }, { type: "step-finish" }]
+
+    await hook.event({
+      event: {
+        type: "session.compacted",
+        properties: { sessionID },
+      },
+    })
+
+    // when
+    await hook.event(buildAssistantUpdate({ sessionID, id: "msg_1", parts: stepOnlyParts }))
+    await hook.event(buildAssistantUpdate({ sessionID, id: "msg_2", parts: stepOnlyParts }))
+
+    // then
+    expect(ctx.client.session.messages).not.toHaveBeenCalled()
+    expect(ctx.client.session.summarize).not.toHaveBeenCalled()
+  })
 })