Branch picker should show local branches, not only remote branches

## Description

When creating a new session in the Copilot CLI app, the branch picker only lists branches from GitHub (remote). Local-only branches that haven't been pushed are not visible.

## Use case

A common developer workflow is to work on a local branch first and push to remote only when the work is done. Currently this workflow is not supported because:

1. You create a local branch and start working
2. You want to open a Copilot CLI session based on that branch
3. The branch doesn't appear in the picker because it hasn't been pushed yet

## Expected behavior

The branch picker should list both local and remote branches, allowing users to base their session on a local branch without having to push it first.

## Current behavior

Only remote (GitHub) branches are shown in the branch picker.

No file or test is named. Start by locating the Copilot CLI session-creation branch picker and trace how its branch data is loaded. Update the picker so local-only and remote branches are both available, then run the relevant CLI tests or manually verify a session can be based on an unpushed local branch.

Written by the indexing model from the issue text.