fix: remove invalid assignees field from issue template frontmatter

The 'assignees: ''' value breaks GitHub's YAML parser, causing
templates to be silently ignored and falling back to blank issue +
discussions. Removed the field entirely since no auto-assignment
is needed.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -3,7 +3,6 @@ name: 🐛 Bug report
 description: Report a bug or unexpected behaviour
 title: '[bug] '
 labels: bug
-assignees: ''
 ---
 
 ## Describe the bug

.github/ISSUE_TEMPLATE/feature_request.md

@@ -3,7 +3,6 @@ name: ✨ Feature request
 description: Suggest an idea, enhancement, UX improvement, or refactor
 title: '[enhancement] '
 labels: enhancement
-assignees: ''
 ---
 
 ## Problem / motivation