Agents Window: Open deep links to Agent Host sessions and chats

[sandy081]

Support deep links to Agent Host sessions and individual chats in the Agents Window.

Verification:

1. In an Agent Host session, use prompts such as:
   • Current session link: Give me a link to the current session.
   • Chat-specific link: Create another chat in this session for a different approach and give me the link to that chat.
   • The first prompt should use get_current_session and surface a session link. The second should use create_chat and surface a link containing the new chat target.
2. Verify session links and chat links render as distinct pills with meaningful labels.
3. Using a screen reader or accessibility inspection, verify each pill has an accessible label that identifies whether it targets a session or a chat.
4. Activate the session pill and verify it opens the correct target session.
5. Activate the chat pill and verify it opens the exact target chat, not merely the session’s default or currently active chat.
6. Repeat while another session is active and verify both links still navigate to the correct targets.

Implemented by #331492.