Branch picker should show local branches, not only remote branches

[jkuoppa]

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.