feat(22-04): ChatMentionPopover component

- Create ChatMentionPopover (200px, opens upward, agent icon + name + role label)
- Agents filtered by query, max 5 visible, No agents found empty state
- 3 skeleton rows loading state, onOpenAutoFocus prevented for textarea focus
- Include agent-role-colors dependency for worktree build

ui/src/components/ChatMentionPopover.test.tsx

@@ -1,8 +1,14 @@
-import { describe, it } from "vitest";
+// @vitest-environment jsdom
+import { describe, it, expect } from "vitest";
 
 describe("ChatMentionPopover", () => {
+  it("exports ChatMentionPopover component", async () => {
+    const mod = await import("./ChatMentionPopover");
+    expect(mod.ChatMentionPopover).toBeDefined();
+  });
+
   it.todo("renders agent list filtered by query string");
   it.todo("shows agent icon, name, and role for each item");
-  it.todo("calls onSelect with @agentName on item click");
+  it.todo("calls onSelect with agent name on item click");
   it.todo("shows 'No agents found' when filter matches nothing");
 });

ui/src/components/ChatMentionPopover.tsx

@@ -0,0 +1,73 @@
+import { Popover, PopoverContent, PopoverTrigger } from "@/components/ui/popover";
+import { Skeleton } from "@/components/ui/skeleton";
+import { AgentIcon } from "./AgentIconPicker";
+import { agentRoleColors, agentRoleColorDefault } from "../lib/agent-role-colors";
+import type { Agent, AgentRole } from "@paperclipai/shared";
+
+interface ChatMentionPopoverProps {
+  open: boolean;
+  onOpenChange: (open: boolean) => void;
+  onSelect: (agentName: string) => void;
+  query: string;
+  agents: Agent[];
+  isLoading?: boolean;
+  children: React.ReactNode;
+}
+
+export function ChatMentionPopover({
+  open,
+  onOpenChange,
+  onSelect,
+  query,
+  agents,
+  isLoading,
+  children,
+}: ChatMentionPopoverProps) {
+  const filtered = agents.filter((a) =>
+    a.name.toLowerCase().includes(query.toLowerCase()),
+  );
+
+  return (
+    <Popover open={open} onOpenChange={onOpenChange}>
+      <PopoverTrigger asChild>{children}</PopoverTrigger>
+      <PopoverContent
+        className="w-[200px] p-1"
+        align="start"
+        side="top"
+        onOpenAutoFocus={(e) => e.preventDefault()}
+      >
+        {isLoading ? (
+          <div className="space-y-1 p-1">
+            <Skeleton className="h-7 w-full" />
+            <Skeleton className="h-7 w-full" />
+            <Skeleton className="h-7 w-full" />
+          </div>
+        ) : filtered.length === 0 ? (
+          <p className="text-sm text-muted-foreground text-center py-2">No agents found</p>
+        ) : (
+          <div className="max-h-[180px] overflow-auto">
+            {filtered.slice(0, 5).map((agent) => {
+              const colorClass = agent.role
+                ? (agentRoleColors[agent.role as AgentRole] ?? agentRoleColorDefault)
+                : agentRoleColorDefault;
+              return (
+                <button
+                  key={agent.id}
+                  className="flex items-center gap-2 w-full rounded-sm px-2 py-1.5 text-left text-sm hover:bg-accent hover:text-accent-foreground cursor-pointer"
+                  onClick={() => {
+                    onSelect(agent.name);
+                    onOpenChange(false);
+                  }}
+                >
+                  <AgentIcon icon={agent.icon} className={`h-3.5 w-3.5 ${colorClass}`} />
+                  <span className="truncate">{agent.name}</span>
+                  <span className="ml-auto text-[11px] text-muted-foreground">{agent.role}</span>
+                </button>
+              );
+            })}
+          </div>
+        )}
+      </PopoverContent>
+    </Popover>
+  );
+}