Fix PR Probe result comments, update changelog

[MDA2AV]

Root cause: pull_request events from forks always get a read-only
GITHUB_TOKEN, regardless of the permissions: block. So gh pr comment fails for
external contributors.

Fix — two-workflow pattern:

1. probe.yml — Instead of commenting directly, it now saves pr-number.txt and
   probe-comment.md as an artifact (probe-pr-comment). Removed the pull-requests:
   write permission since it no longer needs it.
2. probe-comment.yml (new) — Triggers on workflow_run completion of the
   "Probe" workflow. Since workflow_run always runs in the base repo context, its
   GITHUB_TOKEN has full write permissions. It downloads the artifact and
   posts/updates the comment.

This is the GitHub-recommended pattern for posting PR comments from fork PRs.