fix(coding-agent): clear tree navigation compaction state

closes #3688

packages/coding-agent/test/suite/regressions/3688-tree-cancel-compacting.test.ts

@@ -0,0 +1,36 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { assistantMsg, userMsg } from "../../utilities.js";
+import { createHarness, type Harness } from "../harness.js";
+
+describe("issue #3688 tree cancellation compaction state", () => {
+	const harnesses: Harness[] = [];
+
+	afterEach(() => {
+		while (harnesses.length > 0) {
+			harnesses.pop()?.cleanup();
+		}
+	});
+
+	it("clears branch summary state when session_before_tree cancels navigation", async () => {
+		const harness = await createHarness({
+			extensionFactories: [
+				(pi) => {
+					pi.on("session_before_tree", () => ({ cancel: true }));
+				},
+			],
+		});
+		harnesses.push(harness);
+
+		const targetId = harness.sessionManager.appendMessage(userMsg("first"));
+		harness.sessionManager.appendMessage(assistantMsg("reply"));
+		const currentLeafId = harness.sessionManager.appendMessage(userMsg("second"));
+
+		expect(harness.sessionManager.getLeafId()).toBe(currentLeafId);
+
+		const result = await harness.session.navigateTree(targetId, { summarize: false });
+
+		expect(result).toEqual({ cancelled: true });
+		expect(harness.session.isCompacting).toBe(false);
+		expect(harness.sessionManager.getLeafId()).toBe(currentLeafId);
+	});
+});