new: dev: Enable PR-Agent reviews on merge requests

Adds a CI job that runs PR-Agent against each merge request opened
from the canonical repository, posting an automated review and
code-improvement suggestions as MR comments. The job is gated to
same-project source branches so the OpenAI key and personal access
token are not exposed to fork pipelines.

Merge branch 'ondrej/add-pr-agent' into 'main'

See merge request isc-projects/bind9!12032