feat(arena): Add agent collaboration arena feature

Introduces a new Arena system for running multiple AI agents in parallel
terminal sessions with support for iTerm and Tmux backends.

Core:
- Add ArenaManager and ArenaAgentClient for orchestrating multi-agent sessions
- Add terminal backends (ITermBackend, TmuxBackend) with feature detection
- Add git worktree service for isolated agent workspaces
- Add arena event system for real-time status updates

CLI:
- Add /arena command with start, stop, status, and select subcommands
- Add Arena dialogs (Select, Start, Status, Stop)
- Add ArenaCards component for displaying parallel agent outputs
- Consolidate message components into StatusMessages and ConversationMessages
- Add MultiSelect component for agent selection

Config:
- Add arena-related settings to schema and config

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/ui/hooks/useGeminiStream.test.tsx

@@ -217,6 +217,7 @@ describe('useGeminiStream', () => {
         .fn()
         .mockReturnValue(contentGeneratorConfig),
       getMaxSessionTurns: vi.fn(() => 50),
+      getArenaAgentClient: vi.fn(() => null),
     } as unknown as Config;
     mockOnDebugMessage = vi.fn();
     mockHandleSlashCommand = vi.fn().mockResolvedValue(false);