Add GitLab issue and merge request templates
## Description
The AI module currently has no structured templates for GitLab work items or merge requests. Contributors must write issue descriptions and MR descriptions from scratch, leading to inconsistent quality and missing information that slows down triage and review.
A set of templates was prepared as a foundation and then adapted to align with [Drupal's issue etiquette guidelines](https://www.drupal.org/docs/develop/issues/issue-procedures-and-etiquette/issue-etiquette) and [Drupal's AI contribution policy](https://www.drupal.org/docs/develop/issues/issue-procedures-and-etiquette/policy-on-the-use-of-ai-when-contributing-to-drupal).
The templates use GitLab Flavored Markdown hidden comments (`<!-- -->`) to guide contributors without cluttering the rendered view. They include inline hints for GitLab quick actions (`/label`, `/relate`, `/blocks`, `/duplicate`, `/subscribe`) and a `ddev`-based setup snippet for local reproduction on a clean install.
Five issue templates and one merge request template are ready to be committed:
| Template | Type |
|---|---|
| `bug_report.md` | Bug report |
| `feature_request.md` | Feature request |
| `task.md` | Task |
| `plan.md` | Plan / RFC |
| `support_request.md` | Support request |
| `Default.md` (MR) | Merge request |
## Tasks
* [ ] Commit `.gitlab/issue_templates/` containing the five issue templates
* [ ] Commit `.gitlab/merge_request_templates/Default.md`
* [ ] Open a merge request against `1.x`
* [ ] Verify all templates appear in the GitLab work item template picker
* [ ] Confirm `/label` quick actions apply the correct labels on issue creation
## Acceptance criteria
* Selecting "New issue" in GitLab presents all five issue templates as choices
* Each template pre-fills the correct `/label` quick action for its type
* The `Default` MR template appears when opening a new merge request
* All guidance comments are hidden in the rendered view
## Testing instructions *(added by implementor before review)*
1. Navigate to the project's work items and select "New issue"
2. Verify all five templates appear in the template picker
3. Select each template and confirm structure, quick actions, and hidden comments render correctly
4. Open a new merge request and verify the `Default` template is pre-filled
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**Note:** This issue description was drafted with significant AI assistance and has been reviewed by the author. See [Drupal's AI contribution policy](https://www.drupal.org/docs/develop/issues/issue-procedures-and-etiquette/policy-on-the-use-of-ai-when-contributing-to-drupal).
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