Keep template subactions unassigned when applied

Action card templates can now keep subactions set to Unassigned. Fixed user, team, and placeholder assignments are preserved, while unassigned template subactions no longer inherit the parent action's assignee.

The behavior also applies to nested template subactions, while manually created subactions keep the existing parent-assignee behavior.

This helps teams using templates for intake and repeatable processes mix fixed ownership with work that should remain available for team members to pick up.