feat: add automation condition to filter private notes (#12102)

## Summary

Adds a new automation condition to filter private notes.

This allows automation rules to explicitly include or exclude private
notes instead of relying on implicit behavior.

Fixes: #11208 

## Preview



https://github.com/user-attachments/assets/c40f6910-7bbf-4e59-aae5-ad408602927a

app/javascript/dashboard/i18n/locale/en/automation.json

@@ -169,6 +169,7 @@
     },
     "ATTRIBUTES": {
       "MESSAGE_TYPE": "Message Type",
+      "PRIVATE_NOTE": "Private Note",
       "MESSAGE_CONTAINS": "Message Contains",
       "EMAIL": "Email",
       "INBOX": "Inbox",