fix(copilot): explain review request denials instead of forwarding a bare 404

The review request endpoint requires write access to the repository, and
GitHub refuses a caller without it with 404 Not Found rather than a
permission error. Authoring the pull request does not grant that access,
so a fork contributor can be offered a Copilot review by the web UI and
still be refused by request_copilot_review, with nothing in the tool
result to say why.

On 403 or 404 the tool now reads the repository once so it can name the
cause. A caller without write access is told so directly and pointed at
the web UI. When the repository cannot be read at all, or when write
access is present, the message says so and points at the likelier
cause.