Automation: composer.json change → needs-dependency-review + ping marcus and artem
## Summary
When an MR changes `composer.json` (root or sub-module), add the `needs-dependency-review` label and post a comment pinging marcus and artem.
## Work Track
Cross-track (AI Core / coordination)
## Context / Background
Dependency changes need an extra eye for license / supply chain reasons. Detect any `composer.json` change in MR diffs and route it to the dependency reviewers automatically.
Parent / related: #3586474.
This issue is blocked by the three exploration issues (#3586475, #3586476, #3586477) — the platform choice and the available `gitlab_api` primitives must land first.
## Tasks
* [ ] Subscribe to `Merge Request Hook` events (open + update).
* [ ] Inspect the diff: if any `composer.json` (root or sub-module) is added / modified, set `needs-dependency-review` on the MR (and on the linked issue).
* [ ] Post a single comment pinging marcus and artem (replace with their drupal.org / GitLab usernames during implementation) explaining what changed.
* [ ] Edit (do not re-post) the comment on subsequent updates.
* [ ] When all `composer.json` changes are reverted, remove the label.
## Acceptance Criteria
* [ ] An MR that touches `composer.json` is labelled and the reviewers are pinged exactly once.
* [ ] Reverted changes clear the label.
* [ ] No duplicate ping comments.
## Related issues
## AI Usage
- [x] AI Assisted Issue — This issue was generated with AI assistance, but was reviewed and refined by the creator.
issue