`mode` deprecated, use `agent` instead

[hlineholm-teledyne]

Prompt file headers have changed, mode should be agent. In the Additional Resources of the README.md, when navigating to Custom Chat Modes you're redirected to Custom Agents. References to "chat modes" should probably be changed to "custom agents" or "agent mode" throughout this repository to avoid confusion and comments from Copilot code reviews saying agent: agent should be changed to mode: agent.