Move reviewer/approver pickers to inline header pills

Extract execution participant pickers from sidebar PropertyPicker rows into
compact pill-style Popover triggers in the issue header row, next to labels.
Creates a reusable ExecutionParticipantPicker component with matching
text-[10px] sizing. Removes the old sidebar rows and unused code.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/pages/IssueDetail.tsx

@@ -43,6 +43,7 @@ import { InlineEditor } from "../components/InlineEditor";
 import { CommentThread } from "../components/CommentThread";
 import { IssueDocumentsSection } from "../components/IssueDocumentsSection";
 import { IssueProperties } from "../components/IssueProperties";
+import { ExecutionParticipantPicker } from "../components/ExecutionParticipantPicker";
 import { IssueWorkspaceCard } from "../components/IssueWorkspaceCard";
 import { LiveRunWidget } from "../components/LiveRunWidget";
 import type { MentionOption } from "../components/MarkdownEditor";
@@ -1355,6 +1356,23 @@ export function IssueDetail() {
             </div>
           )}
 
+          <div className="flex items-center gap-1">
+            <ExecutionParticipantPicker
+              issue={issue}
+              stageType="review"
+              agents={agents ?? []}
+              currentUserId={currentUserId}
+              onUpdate={(data) => updateIssue.mutate(data)}
+            />
+            <ExecutionParticipantPicker
+              issue={issue}
+              stageType="approval"
+              agents={agents ?? []}
+              currentUserId={currentUserId}
+              onUpdate={(data) => updateIssue.mutate(data)}
+            />
+          </div>
+
           <div className="ml-auto flex items-center gap-0.5 md:hidden shrink-0">
             <Button
               variant="ghost"