fix: update /help test to expect botCommand dispatch, fix PTY fd leak in claude_code.rs (#451, #452)

The /help test expected the help overlay to appear, but /help now goes
through botCommand like other slash commands. Updated the test to match.

Also added reader thread join and child.wait() calls to
claude_code.rs to prevent PTY master fd leaks from web UI chat sessions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/Chat.test.tsx

@@ -1642,7 +1642,8 @@ describe("Slash command handling (Story 374)", () => {
 		expect(mockedApi.botCommand).not.toHaveBeenCalled();
 	});
 
-	it("AC: /help shows help overlay", async () => {
+	it("AC: /help calls botCommand and displays response", async () => {
+		mockedApi.botCommand.mockResolvedValue({ response: "Available commands: status, help, ..." });
 		render(<Chat projectPath="/tmp/project" onCloseProject={vi.fn()} />);
 		await waitFor(() => expect(capturedWsHandlers).not.toBeNull());
 
@@ -1658,9 +1659,14 @@ describe("Slash command handling (Story 374)", () => {
 			fireEvent.keyDown(input, { key: "Enter", shiftKey: false });
 		});
 
-		expect(await screen.findByTestId("help-overlay")).toBeInTheDocument();
+		await waitFor(() => {
+			expect(mockedApi.botCommand).toHaveBeenCalledWith(
+				"help",
+				"",
+				undefined,
+			);
+		});
 		expect(lastSendChatArgs).toBeNull();
-		expect(mockedApi.botCommand).not.toHaveBeenCalled();
 	});
 
 	it("AC: botCommand API error shows error message in chat", async () => {