[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.test.tsx

@@ -753,6 +753,19 @@ describe("MarkdownEditor", () => {
     });
   });
 
+  it("marks the autocomplete portal as floating UI for modal pointer handling", async () => {
+    const handleChange = vi.fn();
+    const { option, root } = await openMentionMenuFor(handleChange);
+
+    const menu = option.closest("[data-paperclip-floating-ui]");
+    expect(menu).toBeTruthy();
+    expect(menu?.className).toContain("pointer-events-auto");
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
   it("does not preventDefault on touchstart so the mention menu can scroll on mobile", async () => {
     const handleChange = vi.fn();
     const { option, root } = await openMentionMenuFor(handleChange);