Allow more control over PR comment resolution trailers

### Description

Users should have more control over the attribution trailer added when Copilot resolves a PR review comment, including its wording, presentation, and whether it is included.

### Example

When a user identifies a review comment and explicitly asks Copilot to address it, the resulting comment currently includes a prominent note stating:

> Auto-resolved by the GitHub Copilot app.

In this case, “auto-resolved” is inaccurate because the user initiated the resolution. The note and icon are also unnecessarily prominent; a concise one-line trailer would be sufficient.

### Expected behavior

The trailer should accurately distinguish between user-prompted and agent merge PR comments. Its formatting should remain lightweight, with an option to customize or disable it.

该 issue 未指定任何文件、测试或入口点；首先定位 Copilot 的 PR 评论解决流程，以及生成其归属标记尾注的位置。完成标准是区分由用户发起的解决和由代理发起的解决，展示保持轻量，并且用户可以自定义或禁用该尾注。

由索引模型根据 Issue 内容生成。