feat: Add 'None' option to remove assigned agents/teams in macros and automations

- Add 'None' option (with id: 'nil') to agent and team assignment dropdowns in macros
- Deprecate remove_assigned_team action in favor of assign_team with 'None' option
- Update helper functions to properly display 'None' for unassignment
- Add comprehensive test coverage for the new functionality
- Maintain backward compatibility with existing remove_assigned_team action

Fixes #7551

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/javascript/dashboard/composables/useMacros.js

@@ -23,10 +23,18 @@ export const useMacros = () => {
   const getMacroDropdownValues = type => {
     switch (type) {
       case 'assign_team':
+        return [
+          { id: 'nil', name: t('AUTOMATION.NONE_OPTION') },
+          ...teams.value,
+        ];
       case 'send_email_to_team':
         return teams.value;
       case 'assign_agent':
-        return [{ id: 'self', name: 'Self' }, ...agents.value];
+        return [
+          { id: 'nil', name: t('AUTOMATION.NONE_OPTION') },
+          { id: 'self', name: 'Self' },
+          ...agents.value,
+        ];
       case 'add_label':
       case 'remove_label':
         return labels.value.map(i => ({