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/components/MarkdownBody.test.tsx

@@ -316,12 +316,16 @@ describe("MarkdownBody", () => {
     expect(html).toContain('rel="noreferrer"');
   });
 
-  it("prefixes GitHub markdown links with the GitHub icon", () => {
+  it("prefixes GitHub markdown links with the GitHub icon glued to the first character", () => {
     const html = renderMarkdown("[https://github.com/paperclipai/paperclip/pull/4099](https://github.com/paperclipai/paperclip/pull/4099)");
 
     expect(html).toContain('<a href="https://github.com/paperclipai/paperclip/pull/4099"');
     expect(html).toContain('class="lucide lucide-github mr-1 inline h-3.5 w-3.5 align-[-0.125em]"');
-    expect(html).toContain(">https://github.com/paperclipai/paperclip/pull/4099</a>");
+    // The icon and first character "h" must sit in a no-wrap span so the
+    // icon can never be orphaned on the previous line from the URL text.
+    expect(html).toMatch(/<span style="white-space:nowrap">.*lucide-github.*?<\/svg>h<\/span>/);
+    expect(html).toContain("ttps://github.com/paperclipai/paperclip/pull/4099");
+    expect(html).not.toContain("lucide-external-link");
   });
 
   it("prefixes GitHub autolinks with the GitHub icon", () => {
@@ -338,6 +342,22 @@ describe("MarkdownBody", () => {
     expect(html).not.toContain("lucide-github");
   });
 
+  it("suffixes external links with a new-tab icon glued to the last character", () => {
+    const html = renderMarkdown("[docs](https://example.com/docs)");
+
+    expect(html).toContain('target="_blank"');
+    expect(html).toContain("lucide-external-link");
+    // Last character "s" must sit in a no-wrap span with the icon so the
+    // indicator never wraps away from the link text.
+    expect(html).toMatch(/<span style="white-space:nowrap">s<svg[^>]*lucide-external-link/);
+  });
+
+  it("does not render the new-tab icon on internal links", () => {
+    const html = renderMarkdown("[settings](/company/settings)");
+
+    expect(html).not.toContain("lucide-external-link");
+  });
+
   it("keeps fenced code blocks width-bounded and horizontally scrollable", () => {
     const html = renderMarkdown("```text\nGET /heartbeat-runs/ca5d23fc-c15b-4826-8ff1-2b6dd11be096/log?offset=2062357&limitBytes=256000\n```");