fix(cli): address Gemini test report — local mode, validate bug, offline flag

A. acr init --local: new local mode, no Cloudflare account required;
   config defaults to mode:local when ~/.arcrun/config.yaml missing

B. validate node-count bug: removed faulty input/output node heuristic
   that dropped start/end nodes from config check; now all nodes except
   reserved 'input' keyword must have config entries

C. acr validate --offline: skip remote component-existence and credentials
   checks; local mode also auto-skips these checks

D. parts.ts: replace require('node:fs') with static import (ES module fix)

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

cli/src/lib/cf-api.ts

@@ -87,7 +87,7 @@ export async function encryptCredential(value: string, encryptionKey: string): P
   const keyBytes = hexToUint8Array(encryptionKey);
   const cryptoKey = await crypto.subtle.importKey(
     'raw',
-    keyBytes,
+    keyBytes.buffer as ArrayBuffer,
     { name: 'AES-GCM' },
     false,
     ['encrypt'],