ci: don't crash the Claude workflow on a fork pull request

GitHub withholds secrets from a workflow triggered by a pull request from
a fork and hands it a read-only token, so the per-actor token never
resolves there. The step that explains a missing token then died with a
403, because posting a comment needs a write token too.

Report through the log when commenting isn't possible, and say that a fork
is the likely reason, so the run ends with something legible instead of an
unhandled HttpError.