Expand the Triage role (edit issue title/body, transfer, convert) [GA]

### Value Prop
The triage repository role now supports editing issue titles and descriptions, transferring issues between repositories, and converting issues to discussions — giving trusted contributors the tools they need to actually manage a backlog. You can bring on community triagers without handing over push access to your codebase, keeping your source code secure while still getting real help with housekeeping. This makes the triage role genuinely useful for open source maintainers and teams that rely on community contributions.

### Expected Outcome
Maintainers should be able to delegate issue and project housekeeping to trusted contributors without granting write access to source code. By closing the gap between what the triage role is named for and what it can actually do, teams can scale their issue management safely and with greater confidence. This reduces the friction of growing an open source project or a large team-managed repository.

The issue names no files, tests, or entry points, so first locate the triage-role permission implementation and its related tests. Done means triage members can edit issue titles and bodies, transfer issues, and convert them to discussions without receiving source-code push access.

索引モデルが issue の本文から書いたものです。