[9.20] 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.

Backport of MR!12032, MR!12033 and MR!12035

Merge branch 'ondrej/add-pr-agent-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12034