fix(views): restore issue-mention class on <a> for mention card

PR #1502's IssueChip extraction moved the `issue-mention` class from the
outer <a> into IssueChip's inner <span>, breaking three consumers that
select on `<a>.issue-mention` directly:

- `.rich-text-editor a.issue-mention` underline-exemption in
  content-editor.css (stopped matching -> mentions in editor gained a
  spurious underline).
- `link-hover-card.tsx` classList check that suppresses the URL preview
  on issue mentions (stopped matching -> hover card wrongly pops up
  over mention chips).
- Tailwind Typography prose (`prose a { text-decoration: underline }`)
  covers a separate path — markdown bubbles in chat. prose's specificity
  (0,1,1) beats `.no-underline` (0,1,0), so `not-prose` is the right
  escape hatch on the AppLink.

Put `issue-mention` back on the <a> in both wrappers (IssueMentionCard
and the editor's MentionView), and add `not-prose` only to the markdown
wrapper. IssueChip's BASE_CLASS keeps `issue-mention` too (inert on the
span; removing it is a separate scope that needs a full consumer audit).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/views/editor/extensions/mention-view.tsx

@@ -60,7 +60,7 @@ function IssueMention({
   };
 
   return (
-    <a href={issuePath} onClick={handleClick} className="inline-flex">
+    <a href={issuePath} onClick={handleClick} className="issue-mention inline-flex">
       <IssueChip
         issueId={issueId}
         fallbackLabel={fallbackLabel}

packages/views/issues/components/issue-mention-card.tsx

@@ -18,7 +18,7 @@ interface IssueMentionCardProps {
 export function IssueMentionCard({ issueId, fallbackLabel }: IssueMentionCardProps) {
   const p = useWorkspacePaths();
   return (
-    <AppLink href={p.issueDetail(issueId)} className="inline-flex">
+    <AppLink href={p.issueDetail(issueId)} className="issue-mention not-prose inline-flex">
       <IssueChip
         issueId={issueId}
         fallbackLabel={fallbackLabel}