refactor: clarify branch name handling in feature dialogs

- Updated comments in AddFeatureDialog and EditFeatureDialog to better explain the logic for determining the final branch name based on the current worktree context.
- Adjusted logic to ensure that an empty string indicates "unassigned" for primary worktrees, while allowing for the use of the current branch when applicable.
- Simplified branch name handling in useBoardActions to reflect these changes.

apps/app/src/components/views/board-view/hooks/use-board-actions.ts

@@ -102,7 +102,8 @@ export function useBoardActions({
     }) => {
       // Simplified: Only store branchName, no worktree creation on add
       // Worktrees are created at execution time (when feature starts)
-      // Empty string means user chose "use current branch" - don't save a branch name
+      // Empty string means "unassigned" (show only on primary worktree) - convert to undefined
+      // Non-empty string is the actual branch name (for non-primary worktrees)
       const finalBranchName = featureData.branchName || undefined;
 
       const newFeatureData = {