chore: use issue types instead of labels for bug/feature templates

Replace `labels: [bug]` and `labels: [enhancement]` with `type: Bug`
and `type: Feature` to leverage GitHub's native issue types already
configured on this repo.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/ISSUE_TEMPLATE/bug_report.yml

 name: Bug Report
 description: Something isn't working as expected
-labels: [bug]
+type: Bug
 body:
   - type: markdown
     attributes:

.github/ISSUE_TEMPLATE/feature_request.yml

 name: Feature Request
 description: Suggest a new feature or improvement
-labels: [enhancement]
+type: Feature
 body:
   - type: markdown
     attributes: