Feature: Enable concurrent prompting of multiple branches via checkbox selection

## Feature Request

Add the ability to prompt multiple branches concurrently from an agent by selecting checkboxes.

## Description

Currently, users need to prompt branches individually. This feature would allow selecting multiple branches via checkboxes and prompting them all at once, improving efficiency when working with multiple branches.

## Proposed Implementation

- Add checkboxes next to each branch in the UI
- Add a bulk action button to prompt all selected branches
- Handle concurrent processing of multiple branch prompts
- Show progress/status for each branch being processed

## Benefits

- Improved workflow efficiency
- Better user experience when working with multiple branches
- Reduced time spent on repetitive actions

## Acceptance Criteria

- [ ] Users can select multiple branches using checkboxes
- [ ] Selected branches can be prompted concurrently
- [ ] Progress is shown for each branch being processed
- [ ] Error handling for individual branch failures

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

No files, tests, or entry points are named. Start by locating the branch list UI and the existing single-branch prompting flow, then trace how status and errors are reported. Done means multi-selection, concurrent prompts, per-branch progress, and isolated failure handling all work together.

Written by the indexing model from the issue text.