feat(ui): task ID in task-detail breadcrumb header

Render the task identifier (e.g. ) in gray monospace between the
status glyph and the title in the breadcrumb bar. Threads an optional
identifier through Breadcrumb + breadcrumbsEqual, adds a CrumbIdentifier
helper to BreadcrumbBar (font-mono text-muted-foreground design tokens),
and wires issue?.identifier onto the task crumb in IssueDetail.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ui/src/pages/IssueDetail.tsx

@@ -2009,6 +2009,7 @@ export function IssueDetail() {
     [comments, optimisticComments],
   );
   const breadcrumbTitle = issue?.title ?? issueId ?? "Task";
+  const breadcrumbIdentifier = issue?.identifier ?? issueHeaderSeed?.identifier ?? undefined;
   const breadcrumbStatus = issue?.status;
   const breadcrumbBlockerAttention = issue?.blockerAttention;
   // Stable identity for the breadcrumb status glyph. The glyph's shape/colour
@@ -3176,12 +3177,14 @@ export function IssueDetail() {
         // The status glyph (leading) already conveys in-progress/live state;
         // no redundant 🔵 emoji prefix on the title.
         label: breadcrumbTitle,
+        identifier: breadcrumbIdentifier,
         leading: breadcrumbStatusLeading,
         leadingKey: breadcrumbStatusKey,
       },
     ]);
   }, [
     breadcrumbTitle,
+    breadcrumbIdentifier,
     hasLiveRuns,
     setBreadcrumbs,
     sourceBreadcrumb.href,