compaction: Settings overrides + GUI divider (OPE-27 3/4)

Settings -> Models grows a Context compaction card next to Token savings:
the trigger % of the context window (10-95), the absolute token cap
(clamped 10k-2M), and the summarizer-model pin (default: the session's
own model). POST /v1/settings/compaction persists them; engines read the
knobs live per check, so changes apply to running sessions immediately.

The "context compacted" divider rides the existing notice machinery: the
persisted `compacted` notice replays on reload (itemsFromMessages) and
the live COMPACTED event appends the same info notice mid-turn. The
transcript itself stays intact - outbound-only by construction.

Covered by vitest (marker replay), a settings-card e2e (defaults +
clamped POSTs + model pin), and a mid-session divider e2e driven by the
fixtures' scripted `compacted` event.

surfaces/gui/e2e/compaction.spec.ts

@@ -0,0 +1,75 @@
+// OPE-27 — auto-compaction GUI: the Settings card's two overrides + summarizer-model
+// pin POST through, and the "context compacted" divider renders inline mid-session
+// (driven by the fixtures' scripted `compacted` event) without touching the transcript.
+import { expect } from "@playwright/test";
+import { test } from "./fixtures";
+
+test("Settings: Context compaction card edits threshold, cap, and summarizer model", async ({
+  page,
+}) => {
+  await page.goto("/");
+  await page.getByTestId("account-row").click();
+  await page.getByRole("button", { name: "Settings", exact: true }).click();
+  await page.getByRole("button", { name: "Models", exact: true }).click();
+
+  const card = page.getByTestId("compaction-card");
+  await expect(card).toBeVisible();
+  await expect(card.getByText("Context compaction")).toBeVisible();
+
+  // Defaults render when the backend doesn't send the fields (older-backend robustness).
+  await expect(card.getByTestId("compaction-threshold")).toHaveValue("80");
+  await expect(card.getByTestId("compaction-cap")).toHaveValue("250000");
+  await expect(card.getByTestId("compaction-model")).toHaveValue("");
+
+  // Threshold edits POST as a fraction, clamped to 10–95%.
+  const [req] = await Promise.all([
+    page.waitForRequest(
+      (r) => r.url().endsWith("/v1/settings/compaction") && r.method() === "POST",
+    ),
+    card.getByTestId("compaction-threshold").fill("70"),
+  ]);
+  expect(req.postDataJSON()).toEqual({ compaction_threshold_pct: 0.7 });
+
+  const [req2] = await Promise.all([
+    page.waitForRequest(
+      (r) => r.url().endsWith("/v1/settings/compaction") && r.method() === "POST",
+    ),
+    card.getByTestId("compaction-cap").fill("100000"),
+  ]);
+  expect(req2.postDataJSON()).toEqual({ compaction_cap_tokens: 100000 });
+
+  // Summarizer pin: the picker offers the session-default plus the configured models.
+  const [req3] = await Promise.all([
+    page.waitForRequest(
+      (r) => r.url().endsWith("/v1/settings/compaction") && r.method() === "POST",
+    ),
+    card.getByTestId("compaction-model").selectOption("gpt-4o-mini"),
+  ]);
+  expect(req3.postDataJSON()).toEqual({ compaction_model: "gpt-4o-mini" });
+});
+
+test("the compacted divider renders mid-session and the transcript stays intact", async ({
+  page,
+}) => {
+  await page.goto("/");
+  await page.getByText("Draft the launch note").first().click();
+  const box = page.getByPlaceholder(/Ask the coworker/);
+
+  // An earlier exchange that must survive the compaction marker (transcript intact).
+  await box.fill("remember the launch date");
+  await box.press("Enter");
+  await expect(page.getByText("Echo: remember the launch date").first()).toBeVisible({
+    timeout: 10_000,
+  });
+
+  await box.fill("compact the context");
+  await box.press("Enter");
+  await expect(
+    page.getByText("Context compacted — earlier turns were summarized").first(),
+  ).toBeVisible({ timeout: 10_000 });
+  await expect(
+    page.getByText("Still on it — continuing where I left off.").first(),
+  ).toBeVisible();
+  // Outbound-only: everything before the divider is still on screen.
+  await expect(page.getByText("Echo: remember the launch date").first()).toBeVisible();
+});

surfaces/gui/e2e/fixtures.ts

@@ -681,6 +681,14 @@ export async function mockApi(page: import("@playwright/test").Page) {
           }, 120);
           return;
         }
+        // Auto-compaction (OPE-27): the server compacts mid-run and emits the marker,
+        // then the turn continues normally — the divider must render inline.
+        if (/compact the context/i.test(msg.text)) {
+          send("compacted", { text: "Context compacted — earlier turns were summarized" });
+          send("assistant_message", { text: "Still on it — continuing where I left off." });
+          send("turn_done");
+          return;
+        }
         // A turn that dies on a provider error; the follow-up {type:"retry"} recovers.
         if (/fail the turn/i.test(msg.text)) {
           send("error", { error: "model unreachable" });