Improve TXT export readability by using label:value format instead of raw JSON

### Describe your improvement idea or feature request

Currently, exporting tasks as a `.txt` file outputs raw JSON data, which is difficult for users to read and understand.
This enhancement proposes formatting TXT exports in a human-readable `label: value` format while keeping JSON export unchanged.

### How would you like to see it implemented?

1. Exported `.txt` files should display tasks in a readable format such as:
- Description: Buy groceries
- Status: pending
- Due: 2025-01-01 10:00
- Priority: L
- Tags: home, shopping
2. JSON export should remain unchanged and continue to provide raw data.

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### How can we contact you (optional)

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### Would you like to work on this issue?

Yes

### By submitting this issue, I have confirmed that:

- [x] I have starred the repo ⭐ and watched 👀 it on GitHub and followed the contribution guidelines.

タスクのエクスポート実装を特定し、TXT と JSON のエクスポートパスを比較します。TXT 出力のみを更新して、要求された label: value フィールドを使用するようにし、その後 JSON が変更されていないことと、エクスポートされたタスクに一覧の読み取り可能な値が含まれてい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。