fix(runtime): expand bare ~ in XCODEBUILDMCP_CWD

resolveCwdOverride only matched `~/` so a bare `~` was passed to
process.chdir as a literal path and failed with ENOENT. Replace the
inlined logic with the canonical expandHomePrefix() helper from
utils/path.ts (which handles bare `~`, `~/...`, and empty input) and
add a test for the bare-~ case.

Reported by Cursor Bugbot in #374.

Author: cameroncooke

src/runtime/__tests__/bootstrap-runtime.test.ts

@@ -1,5 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import path from 'node:path';
+import os from 'node:os';
 
 const { scheduleSimulatorDefaultsRefreshMock } = vi.hoisted(() => ({
   scheduleSimulatorDefaultsRefreshMock: vi.fn(),
@@ -183,6 +184,12 @@ describe('bootstrapRuntime', () => {
       expect(calledWith.startsWith('~')).toBe(false);
     });
 
+    it('expands a bare ~ to the home directory', async () => {
+      process.env.XCODEBUILDMCP_CWD = '~';
+      await bootstrapRuntime({ runtime: 'cli', fs: createFsWithSessionDefaults() });
+      expect(chdirSpy).toHaveBeenCalledWith(os.homedir());
+    });
+
     it('falls back gracefully when chdir throws', async () => {
       process.env.XCODEBUILDMCP_CWD = '/nonexistent';
       chdirSpy?.mockImplementation(() => {

src/runtime/bootstrap-runtime.ts

@@ -1,5 +1,4 @@
 import process from 'node:process';
-import os from 'node:os';
 import {
   initConfigStore,
   getConfig,
@@ -11,6 +10,7 @@ import { getDefaultFileSystemExecutor } from '../utils/command.ts';
 import { log } from '../utils/logger.ts';
 import type { FileSystemExecutor } from '../utils/FileSystemExecutor.ts';
 import { scheduleSimulatorDefaultsRefresh } from '../utils/simulator-defaults-refresh.ts';
+import { expandHomePrefix } from '../utils/path.ts';
 
 export type RuntimeKind = 'cli' | 'daemon' | 'mcp';
 
@@ -100,10 +100,7 @@ function resolveCwdOverride(): string | undefined {
   if (!raw) {
     return undefined;
   }
-  if (raw.startsWith('~/')) {
-    return `${os.homedir()}${raw.slice(1)}`;
-  }
-  return raw;
+  return expandHomePrefix(raw);
 }
 
 export async function bootstrapRuntime(