[FEATURE]: Clarify organization input instructions

### Feature and its Use Cases

The current helper text says:

> Type an org name and press Enter or comma to add.

It would be clearer to specify that users should enter the **GitHub organization username**.

**Suggested text:**

> Type a GitHub organization username (e.g., `AOSSIE-Org`) and press Enter or comma to add.

This small change would reduce confusion and make the expected input more explicit for new users.

### Additional Context

This is a small UI/UX improvement that only updates the helper text without changing any functionality.

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

No file, test, or entry point is named in the issue. Locate the organization input's helper text in the JavaScript frontend, update it to the suggested wording, and verify that the displayed instruction identifies a GitHub organization username and retains the Enter-or-comma guidance.

索引モデルが issue の本文から書いたものです。