fix: construct full URL when using --token for headless login (#21)

When using `npx atxp login --token <token>`, the raw token was saved
directly to the config file. However, the @atxp/client library expects
a full URL format with the connection_token parameter.

This caused "Invalid URL" errors when trying to use any ATXP tools
after headless login.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: emilioacc

packages/atxp/src/login.ts

@@ -33,7 +33,7 @@ export async function login(options: LoginOptions = {}): Promise<void> {
 
     // If token provided directly, use it (headless mode)
     if (options.token) {
-      connectionString = options.token;
+      connectionString = `https://accounts.atxp.ai?connection_token=${options.token}`;
       console.log('Using provided token for headless authentication...');
     } else if (options.qr) {
       // QR code mode explicitly requested