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.

The issue names no files, tests, or entry points; begin by locating the Copilot PR-comment resolution flow and where its attribution trailer is generated. Done means user-prompted and agent-initiated resolutions are distinguished, the presentation is lightweight, and users can customize or disable the trailer.

Written by the indexing model from the issue text.