Make preview comment sticky

[MattiasBuelens]

Follow-up on #786: replace the actions/github-script step that calls issues.createComment() (a new comment on every push) with marocchino/sticky-pull-request-comment@v2 using header: pr-preview, so each new deployment updates the same comment.

The message is modelled on rossjrw/pr-preview-action's comment, which this repo used before, e.g.

Documentation preview
🚀 View preview at https://docs-preview.optiview.dolby.com/pr-786/
<h6>Deployed from commit abc123… at 2026-08-10 12:00 UTC.</h6>

There's no "preview will be ready once the deployment completes" line: the comment is posted after the S3 sync and CloudFront invalidation, so the preview is already live.

_undeploy.yml now updates the same sticky comment to say the preview was removed, then hides it (minimized as outdated). That takes two steps because the action ignores message when hide: true: one step with only_update: true (a no-op if there is no existing comment) and one with hide: true.

The timestamp comes from a small date -u step (the sticky action takes a plain string, not JS). Both jobs get pull-requests: write, the undeploy job gets the app token (and secrets: inherit in pull-request.yml) so the comment is posted by the bot.

Link to Devin session: https://dolby.devinenterprise.com/sessions/5dd3c46c6c65412c8954ba72ef6d43db
Requested by: @MattiasBuelens

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring