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/composables/spec/useAutomation.spec.js

@@ -8,6 +8,7 @@ import {
   agents,
   teams,
   labels,
+  booleanFilterOptions,
   statusFilterOptions,
   messageTypeOptions,
   priorityOptions,
@@ -73,6 +74,8 @@ describe('useAutomation', () => {
           return countries;
         case 'message_type':
           return messageTypeOptions;
+        case 'private_note':
+          return booleanFilterOptions;
         case 'priority':
           return priorityOptions;
         default:
@@ -226,6 +229,9 @@ describe('useAutomation', () => {
     expect(getConditionDropdownValues('message_type')).toEqual(
       messageTypeOptions
     );
+    expect(getConditionDropdownValues('private_note')).toEqual(
+      booleanFilterOptions
+    );
     expect(getConditionDropdownValues('priority')).toEqual(priorityOptions);
   });