feat(arena): improve agent UI with header info and simplify worktree branches

- Add AgentHeader component showing model, path, and git branch
- Separate modelId and modelName in RegisteredAgent for cleaner display
- Simplify worktree branch naming from worktrees/session/name to base-session-name
- Change loading text from "Agent is working…" to "Thinking…"
- Make agent footer always visible (not just when input is active)

This improves the agent collaboration UX by providing context about each
agent's environment and simplifies the git worktree management.

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

packages/cli/src/ui/hooks/useArenaInProcess.ts

@@ -93,8 +93,9 @@ export function useArenaInProcess(
             actionsRef.current.registerAgent(
               agentState.agentId,
               interactive,
-              agentState.model.displayName || agentState.model.modelId,
+              agentState.model.modelId,
               nextColor(),
+              agentState.model.displayName,
             );
           }
         }
@@ -115,8 +116,9 @@ export function useArenaInProcess(
             actionsRef.current.registerAgent(
               event.agentId,
               interactive,
-              event.model.displayName || event.model.modelId,
+              event.model.modelId,
               nextColor(),
+              event.model.displayName,
             );
             return;
           }