Automate screenshot previews for article PRs

[mateu]

What

Automatically run article screenshot previews for PRs and fix workflow conditions that prevented comments/skips from behaving correctly.

Why

Issue #408 asks for screenshots on new article pull requests, but the current workflow needs a manual label, fails when no article files are changed, and never posts comments due to an event-condition mismatch.

How

• Trigger screenshot workflow on PR lifecycle events for */incoming/*.pod and */articles/*.pod changes.
• Remove label-gated execution and run for non-draft PRs automatically.
• Treat "no changed articles" as a clean skip instead of a failure.
• Gate expensive screenshot/build steps on detected article changes.
• Fix PR comment step condition to run in pull_request_target events.

Testing

• perl -MYAML::XS=LoadFile -e 'LoadFile(".github/workflows/screenshot.yml")'
• prove -lr t

---

Quality Report

Changes: 1 file changed, 14 insertions(+), 13 deletions(-)

Code scan: clean

Tests: skipped

Branch hygiene: 1 issue(s)

• Branch is not pushed to remote

Generated by Kōan post-mission quality pipeline

[oalders]

I think the really tricky part here is actually getting the screenshots into the PR comments. We'd need some kind of an article to accompany the PR so that we can test the workflow.