feat: read pnpm version from devEngines.packageManager field

When no version is specified in the action config or the packageManager
field of package.json, fall back to devEngines.packageManager.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/install-pnpm/run.ts

@@ -57,21 +57,28 @@ async function readTarget(opts: {
   const { version, packageJsonFile, standalone } = opts
   const { GITHUB_WORKSPACE } = process.env
 
-  let packageManager
+  let packageManager: string | undefined
+  let devEngines: { packageManager?: { name?: string; version?: string } } | undefined
 
   if (GITHUB_WORKSPACE) {
     try {
       const content = readFileSync(path.join(GITHUB_WORKSPACE, packageJsonFile), 'utf8');
-      ({ packageManager } = packageJsonFile.endsWith(".yaml")
+      const manifest = packageJsonFile.endsWith(".yaml")
         ? parseYaml(content, { merge: true })
         : JSON.parse(content)
-      )
+      packageManager = manifest.packageManager
+      devEngines = manifest.devEngines
     } catch (error: unknown) {
       // Swallow error if package.json doesn't exist in root
       if (!util.types.isNativeError(error) || !('code' in error) || error.code !== 'ENOENT') throw error
     }
   }
 
+  // If packageManager field is not set, try devEngines.packageManager
+  if (typeof packageManager !== 'string' && devEngines?.packageManager?.name === 'pnpm' && devEngines.packageManager.version) {
+    packageManager = `pnpm@${devEngines.packageManager.version}`
+  }
+
   if (version) {
     if (
       typeof packageManager === 'string' &&
@@ -98,7 +105,8 @@ Otherwise, please specify the pnpm version in the action configuration.`)
     throw new Error(`No pnpm version is specified.
 Please specify it by one of the following ways:
   - in the GitHub Action config with the key "version"
-  - in the package.json with the key "packageManager"`)
+  - in the package.json with the key "packageManager"
+  - in the package.json with the key "devEngines.packageManager"`)
   }
 
   if (!packageManager.startsWith('pnpm@')) {