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

lib/filters/filter_keys.yml

@@ -214,6 +214,12 @@ messages:
     filter_operators:
       - "equal_to"
       - "not_equal_to"
+  private_note:
+    attribute_type: "standard"
+    data_type: "boolean"
+    filter_operators:
+      - "equal_to"
+      - "not_equal_to"
   content:
     attribute_type: "standard"
     data_type: "text"