agentHost: Default worktree branch picker to repo default branch (#309676)

When the isolation picker is 'worktree', the branch picker now defaults
to the repository's default branch (via refs/remotes/origin/HEAD) instead
of the current branch. When isolation is 'folder', the branch picker
shows the current branch as before.

Adds getDefaultBranch() to IAgentHostGitService.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>