Remove jQuery usage from app/assets/javascripts/behaviors/markdown/copy_as_gfm.js
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## Summary
**Files:** `app/assets/javascripts/behaviors/markdown/copy_as_gfm.js` (~231 lines, Medium)
Handles copy-as-GFM and paste-GFM for markdown content. Uses jQuery for document-level delegated events and `e.originalEvent` unwrapping.
## Implementation plan
- Replace `$(document).on('copy'/'paste', selector, handler)` with delegated native listeners using `e.target.closest(selector)`
- Remove `e.originalEvent` references -- native events provide `e.clipboardData` directly
## Test areas
- Copy text from rendered markdown (issue description, comment) and paste into another markdown input
- Copy code blocks, tables, lists from rendered markdown
- Quote reply on issue/MR comments
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