Rename Git branch in session pane via UI

### Feature summary

Add the ability to change the name of the Git branch via UI

### What problem are you trying to solve?

After writing my initial prompt in a new session, the GitHub Copilot App created a new branch / worktree.

I experimented with three sessions and in two instances, the session mentioned to grasp the session's intend and updated the branch name. However, in one of the cases, liebeck/crispy-invention was chosen

It is already possible to change the session name via UI

After "Rename session", can you please add a "Rename branch" (or whatever the correct naming is) as well?

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_