fix(browser): suppressed blank shared startup window

Added Chromium no-startup-window to the broker-owned browser launch so no unowned foreground page survives session tab cleanup.

Covered the resolved Chromium argv and documented the Windows regression.

Fixes #8615

packages/coding-agent/test/tools/browser-launch.test.ts

@@ -4,6 +4,7 @@ import * as fs from "node:fs";
 import * as path from "node:path";
 import {
 	chromiumExecutableProbeForTest,
+	resolveSharedBrowserLaunchSpec,
 	stealthIgnoreDefaultArgsForTest,
 	systemChromiumCandidatesForTest,
 } from "@oh-my-pi/pi-coding-agent/tools/browser/launch";
@@ -44,6 +45,24 @@ describe("browser launch stealth defaults", () => {
 	});
 });
 
+describe("shared browser launch", () => {
+	it("suppresses the broker-owned blank startup window", async () => {
+		const previousExecutable = process.env.PUPPETEER_EXECUTABLE_PATH;
+		process.env.PUPPETEER_EXECUTABLE_PATH = "/test/chrome";
+		try {
+			const launch = await resolveSharedBrowserLaunchSpec({
+				headless: true,
+				userDataDir: "/test/profile",
+			});
+
+			expect(launch?.args).toContain("--no-startup-window");
+		} finally {
+			if (previousExecutable === undefined) delete process.env.PUPPETEER_EXECUTABLE_PATH;
+			else process.env.PUPPETEER_EXECUTABLE_PATH = previousExecutable;
+		}
+	});
+});
+
 const UNGOOGLED_CHROMIUM_FLATPAK_ID = "io.github.ungoogled_software.ungoogled_chromium";
 
 describe("system Chromium candidates", () => {