make folder to house worktrees configurable

Problem

At the moment, repositories and their worktrees need to exist under the same parent folder. This creates a workflow issue for users who use IDEs and other development tools.

For example, I keep my repositories in a directory that I open in IntelliJ IDEA and several other tools. When worktrees are created alongside the repositories, those tools detect each worktree as a separate Git project, cluttering the workspace with additional projects and diffs that I don't want to see.

Proposed Solution

It would be very helpful if the location where worktrees are created could be made configurable.

This would allow users to:

Keep their primary repository directory clean.
Store worktrees in a separate location dedicated to temporary or parallel work.
Avoid IDEs and other tooling automatically detecting every worktree as an additional Git project.

---
| Field | Value |
| --- | --- |
| App version | 1.0.22 |
| OS | macOS 26.4.1 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 2 |

まず、worktree の作成フローとアプリの設定インターフェースを特定します。現在の親フォルダーがどのように選択されているかを追跡し、次に新しい worktree の構成可能な保存先を定義して、リポジトリを worktree とは別に保持できることを検証します。

索引モデルが issue の本文から書いたものです。