Unable to rename branch in plan mode

### Short summary

Plan mode fails to rename the worktree branch and this blocks plan.md from being created

### Affected version or release

v1.1.14

### Installation context

Windows

### What happened?

In plan mode, the session fails to rename the branch name as it reports the branch name tool is unavailable. This leads to the plan not being created until forced to by yet another prompt.

### Steps to reproduce

1. Start session in repo worktree in plan mode
2. Prompt for a task (I used GPT-5.6 Sol Medium reasoning 400k)
3. Notice how it fails to rename branch and therefore won't make changes in the repo workspace
4. Ask it to create the plan anyway
5. Plan created

### Expected behavior

_No response_

### Additional context

まず、plan mode を使ってリポジトリの worktree で Windows v1.1.14 の失敗を再現します。次に、plan mode でのブランチ名変更と branch-name ツールの利用可能性のフローを追跡し、追加の prompt なしでブランチが名前変更され、plan.md が作成されることを確認します。

索引モデルが issue の本文から書いたものです。