Polish markdown external link wrapping (#4447)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The board UI renders agent comments, PR links, issue links, and
operational markdown throughout issue threads
> - Long GitHub and external links can wrap awkwardly, leaving icons
orphaned from the text they describe
> - Small inbox visual polish also helps repeated board scanning without
changing behavior
> - This pull request glues markdown link icons to adjacent link
characters and removes a redundant inbox list border
> - The benefit is cleaner, more stable markdown and inbox rendering for
day-to-day operator review

## What Changed

- Added an external-link indicator for external markdown links.
- Kept the GitHub icon attached to the first link character so it does
not wrap onto a separate line.
- Kept the external-link icon attached to the final link character so it
does not wrap away from the URL/text.
- Added markdown rendering regressions for GitHub and external link icon
wrapping.
- Removed the extra border around the inbox list card.

## Verification

- `pnpm exec vitest run --project @paperclipai/ui
ui/src/components/MarkdownBody.test.tsx`
- `pnpm --filter @paperclipai/ui typecheck`

## Risks

- Low risk. The markdown change is limited to link child rendering and
preserves existing href/target/rel behavior.
- Visual-only inbox polish.

> 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, tool-enabled with
shell/GitHub/Paperclip API access. Context window was not reported by
the 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

---------

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

ui/src/pages/Inbox.tsx

@@ -2122,7 +2122,7 @@ export function Inbox() {
         <>
           {showSeparatorBefore("work_items") && <Separator />}
           <div>
-            <div ref={listRef} className="overflow-hidden rounded-xl border border-border bg-card">
+            <div ref={listRef} className="overflow-hidden rounded-xl bg-card">
               {(() => {
                 const renderInboxIssue = ({
                   issue,