ChatDigi AI Free 1.0.2 update adjustment

What changed:
- Added a private updater to the Free plugin.
- Added a "Check for Update" link in the WordPress Plugins page.
- Free checks: https://chatdigi.ai/updates/free/
- Free does not require a license.
- Added "Upgrade to Pro" link in the plugin action links.
- Included CHATDIGI-REMOTE-UPDATER-index.php.txt for the update server.

Server setup:
1. Create folder: https://chatdigi.ai/updates/free/
2. Upload CHATDIGI-REMOTE-UPDATER-index.php.txt as index.php in that folder.
3. Upload the latest Free ZIP into that same folder.
4. Keep directory browsing disabled.
