fix(#300 ui): flip avatar hierarchy on comments, make launcher visible

Two visual defects in the agent avatar stack (PR #304), missed by the
code-only review:

- The launcher (human) avatar was fully occluded behind the opaque agent
  glyph — the container was exactly the glyph size, so the launcher sat
  underneath it. Enlarge the container by an overhang and vertically center
  the glyph so the launcher peeks out at the bottom-right and stays visible.
- On comments the human creator stayed the PRIMARY avatar and name while the
  stack was crammed into the old badge slot, duplicating the identity and
  failing the "agent is primary" requirement. AgentAvatarStack gains a
  showName prop; with showName=false it now replaces the leading avatar for
  agent comments, and the name slot renders agent.name (+ dimmed
  · launcher.name). Non-agent comments are byte-identical to before;
  history-item keeps the default (names shown).

Tests: add showName=false and external-MCP (no-launcher) coverage, assert
no identity duplication. client tsc clean, 9 tests pass.

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

apps/client/src/components/ui/agent-avatar-stack.test.tsx

@@ -43,6 +43,24 @@ describe("AgentAvatarStack", () => {
     expect(screen.getByText("Alice")).toBeDefined();
   });
 
+  it("showName=false: renders only the avatars, no inline name label", () => {
+    renderStack({
+      agent: { name: "Researcher", emoji: "🔬", avatarUrl: null },
+      launcher: { name: "Alice", avatarUrl: null },
+      aiChatId: "chat-1",
+      showName: false,
+    });
+
+    // The agent glyph is still rendered...
+    expect(screen.getByText("🔬")).toBeDefined();
+    // ...but neither the agent NOR the launcher inline name label is rendered
+    // (they live only in the hover tooltip, which is not mounted in the initial
+    // DOM) — guards against suppressing only the agent name and leaking the
+    // launcher name.
+    expect(screen.queryByText("Researcher")).toBeNull();
+    expect(screen.queryByText("Alice")).toBeNull();
+  });
+
   it("internal chat WITHOUT role: sparkles fallback + 'AI agent' + launcher", () => {
     const { container } = renderStack({
       agent: { name: "AI agent", avatarUrl: null },