[codex] Fix new issue autocomplete pointer selection (#6311)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies.
> - Human operators create and edit issues through modal-heavy board UI
workflows.
> - The new-issue dialog embeds markdown editors that render
autocomplete menus through body-level portals.
> - Radix Dialog treats those portal clicks as outside-dialog pointer
events and prevents their default behavior.
> - That prevention made completion items hard or impossible to select
from inside the new-issue dialog.
> - This pull request marks the markdown editor floating autocomplete
menu as allowed dialog-external UI and extends the dialog
outside-pointer handler to preserve those interactions.
> - The benefit is that users can click/tap autocomplete completions
while keeping the existing modal behavior intact.

## What Changed

- Added a stable `data-paperclip-floating-ui` marker and explicit
pointer event handling to the markdown editor mention/autocomplete
portal.
- Updated the new issue dialog outside-pointer guard so editor
autocomplete portals are handled like Radix popover portals.
- Added regression coverage for markdown editor portal markup and new
issue dialog completion selection behavior.

## Verification

- `pnpm exec vitest run ui/src/components/MarkdownEditor.test.tsx
ui/src/components/NewIssueDialog.test.tsx` passed: 2 files, 38 tests.
- Confirmed the branch is rebased onto current `public-gh/master` before
opening this PR.
- Confirmed the diff does not include `pnpm-lock.yaml` or
`.github/workflows` changes.

## Risks

- Low risk. The change is scoped to allowing pointer events from known
body-level UI portals while keeping other outside-dialog pointer events
under Radix Dialog control.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent with repository tool use and local
command execution. Exact hosted context window is not surfaced in this
runtime.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

Screenshot note: this is an interaction/event-handling fix with no
visible UI change; verification is covered by the focused regression
tests above.

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/components/MarkdownEditor.tsx

@@ -1241,7 +1241,8 @@ export const MarkdownEditor = forwardRef<MarkdownEditorRef, MarkdownEditorProps>
       {mentionActive && filteredMentions.length > 0 && mentionMenuPosition &&
         createPortal(
           <div
-            className="fixed z-[9999] min-w-[180px] max-w-[calc(100vw-16px)] max-h-[208px] overflow-y-auto rounded-md border border-border bg-popover shadow-md"
+            data-paperclip-floating-ui=""
+            className="pointer-events-auto fixed z-[9999] min-w-[180px] max-w-[calc(100vw-16px)] max-h-[208px] overflow-y-auto rounded-md border border-border bg-popover shadow-md"
             style={{
               top: mentionMenuPosition.top,
               left: mentionMenuPosition.left,