feat: implement session agent selection

[yuhp]

Motivation

To support multi-agent interactions within WebClaw. Users need the ability to choose which specialized agent they want to converse with when starting a new session, and to clearly see which agent they are interacting with in historical sessions.

Description

This PR introduces the Session Agent Selection feature, allowing users to select a specific AI Agent for their chat sessions. It dynamically fetches available agents from the backend and displays the selected agent within the chat interface.

Changes Made

• API Enhancements:
  • Added a new /api/agents endpoint (apps/webclaw/src/routes/api/agents.ts) that fetches the list of available agents via gatewayRpc('agents.list', {}).
  • Updated the /api/sessions.ts endpoint to accept an agentId in the POST body. The agentId is now encoded directly into the session key (agent:<agentId>:<uuid>) instead of being passed as a separate parameter to the Gateway.
• Frontend Updates:
  • Queries: Added fetchAgents to chat-queries.ts to retrieve the agent list and handle fallback naming (using id if name is empty).
  • State & UI: Updated chat-screen.tsx to manage the selectedAgentId state and apply default agent settings. Added a query to fetch the available agents for new chats.
  • Header Selection: Modified chat-header.tsx to display an agent <select> dropdown for new, empty chats. Once a session is established, the dropdown is replaced with a static label (e.g., "Agent: product_manager") displaying the agent associated with that specific session.
  • Utils: Created a deriveAgentIdFromKey utility function in utils.ts to correctly parse the agentId from the modified session key format.

Testing

• Verified that the /api/agents endpoint correctly returns the agent list.
• Confirmed that new chats allow agent selection via the dropdown in the header.
• Ensured that the selected agentId is correctly sent in the session creation payload and encoded into the session key.
• Verified that existing sessions correctly parse their key and display the correct static agent name in the header.