fix: handle Windows with .cmd/.ps1 shims and add tests

- Extract ensureAliasLinks to its own module for testability
- On Windows, create .cmd and .ps1 shims instead of symlinks
- On Unix, create symlinks (as before)
- Skip alias creation when targets don't exist (pnpm v10)
- Add vitest and 8 tests covering unix/windows/skip/no-overwrite

src/install-pnpm/ensureAliasLinks.ts

@@ -0,0 +1,62 @@
+import { writeFile, symlink } from 'fs/promises'
+import { existsSync } from 'fs'
+import path from 'path'
+
+interface AliasDefinition {
+  name: string
+  target: string
+}
+
+function getAliases (standalone: boolean): AliasDefinition[] {
+  if (standalone) {
+    return [
+      { name: 'pn', target: path.join('..', '@pnpm', 'exe', 'pn') },
+      { name: 'pnpx', target: path.join('..', '@pnpm', 'exe', 'pnpx') },
+      { name: 'pnx', target: path.join('..', '@pnpm', 'exe', 'pnx') },
+    ]
+  }
+  return [
+    { name: 'pn', target: path.join('..', 'pnpm', 'bin', 'pnpm.cjs') },
+    { name: 'pnpx', target: path.join('..', 'pnpm', 'bin', 'pnpx.cjs') },
+    { name: 'pnx', target: path.join('..', 'pnpm', 'bin', 'pnpx.cjs') },
+  ]
+}
+
+function cmdShim (target: string): string {
+  return `@ECHO off\r\n"%~dp0\\${target}" %*\r\n`
+}
+
+function pwshShim (target: string): string {
+  return `#!/usr/bin/env pwsh\n& "$PSScriptRoot\\${target}" @args\n`
+}
+
+/**
+ * Create pn/pnpx/pnx alias links in the bin directory.
+ * On Unix, creates symlinks. On Windows, creates .cmd and .ps1 shims.
+ * Only creates links when the target file actually exists (pnpm v11+).
+ */
+export async function ensureAliasLinks (binDir: string, standalone: boolean, platform: NodeJS.Platform = process.platform): Promise<void> {
+  const aliases = getAliases(standalone)
+  const isWindows = platform === 'win32'
+
+  for (const { name, target } of aliases) {
+    const resolvedTarget = path.resolve(binDir, target)
+    if (!existsSync(resolvedTarget)) continue
+
+    if (isWindows) {
+      const cmdPath = path.join(binDir, `${name}.cmd`)
+      if (!existsSync(cmdPath)) {
+        await writeFile(cmdPath, cmdShim(target))
+      }
+      const ps1Path = path.join(binDir, `${name}.ps1`)
+      if (!existsSync(ps1Path)) {
+        await writeFile(ps1Path, pwshShim(target))
+      }
+    } else {
+      const link = path.join(binDir, name)
+      if (!existsSync(link)) {
+        await symlink(target, link)
+      }
+    }
+  }
+}