fix(coding-agent): run extension cleanup and restore terminal on signal exits

Signal-triggered shutdown (SIGTERM/SIGHUP) now emits session_shutdown
before any terminal writes, and SIGHUP no longer hard-exits, so extension
resources (e.g. sockets) are released and the terminal is restored even
when the terminal is gone. A genuinely dead terminal still exits without
hot-spinning via the EIO error path.

closes #5080

packages/coding-agent/test/suite/regressions/5080-signal-shutdown-extension-cleanup.test.ts

@@ -0,0 +1,103 @@
+import { afterEach, describe, expect, test, vi } from "vitest";
+import { InteractiveMode } from "../../../src/modes/interactive/interactive-mode.ts";
+
+// Regression for https://github.com/earendil-works/pi/issues/5080
+//
+// On SIGTERM/SIGHUP the graceful shutdown must emit `session_shutdown`
+// (runtimeHost.dispose) BEFORE touching the terminal. Extension teardown such
+// as removing a socket does not write to the tty, so it must not be skipped if
+// a later terminal-restore write fails on a dead or stalled terminal. The
+// interactive quit path (Ctrl+D, /quit) keeps the opposite order to preserve
+// the final TUI frame.
+
+type ShutdownThis = {
+	isShuttingDown: boolean;
+	unregisterSignalHandlers: () => void;
+	runtimeHost: { dispose: () => Promise<void> };
+	ui: { terminal: { drainInput: (ms: number) => Promise<void> } };
+	stop: () => void;
+};
+
+type InteractiveModePrototypeWithShutdown = {
+	shutdown(this: ShutdownThis, options?: { fromSignal?: boolean }): Promise<void>;
+};
+
+const interactiveModePrototype = InteractiveMode.prototype as unknown;
+
+class ProcessExitError extends Error {}
+
+function createContext(order: string[]): ShutdownThis {
+	return {
+		isShuttingDown: false,
+		unregisterSignalHandlers: vi.fn(),
+		runtimeHost: {
+			dispose: vi.fn(async () => {
+				order.push("dispose");
+			}),
+		},
+		ui: {
+			terminal: {
+				drainInput: vi.fn(async () => {
+					order.push("drainInput");
+				}),
+			},
+		},
+		stop: vi.fn(() => {
+			order.push("stop");
+		}),
+	};
+}
+
+async function callShutdown(context: ShutdownThis, options?: { fromSignal?: boolean }): Promise<void> {
+	try {
+		await (interactiveModePrototype as InteractiveModePrototypeWithShutdown).shutdown.call(context, options);
+	} catch (error) {
+		if (!(error instanceof ProcessExitError)) throw error;
+	}
+}
+
+describe("InteractiveMode.shutdown ordering (#5080)", () => {
+	afterEach(() => {
+		vi.restoreAllMocks();
+	});
+
+	test("signal-triggered shutdown emits session_shutdown before terminal writes", async () => {
+		vi.spyOn(process, "exit").mockImplementation((() => {
+			throw new ProcessExitError();
+		}) as typeof process.exit);
+		const order: string[] = [];
+		const context = createContext(order);
+
+		await callShutdown(context, { fromSignal: true });
+
+		expect(order).toEqual(["dispose", "drainInput", "stop"]);
+		expect(context.isShuttingDown).toBe(true);
+		expect(context.unregisterSignalHandlers).toHaveBeenCalledTimes(1);
+	});
+
+	test("interactive quit stops the TUI before emitting session_shutdown", async () => {
+		vi.spyOn(process, "exit").mockImplementation((() => {
+			throw new ProcessExitError();
+		}) as typeof process.exit);
+		const order: string[] = [];
+		const context = createContext(order);
+
+		await callShutdown(context);
+
+		expect(order).toEqual(["drainInput", "stop", "dispose"]);
+	});
+
+	test("re-entrant shutdown is a no-op", async () => {
+		vi.spyOn(process, "exit").mockImplementation((() => {
+			throw new ProcessExitError();
+		}) as typeof process.exit);
+		const order: string[] = [];
+		const context = createContext(order);
+		context.isShuttingDown = true;
+
+		await callShutdown(context, { fromSignal: true });
+
+		expect(order).toEqual([]);
+		expect(context.runtimeHost.dispose).not.toHaveBeenCalled();
+	});
+});