Yes, it can't change between edit and ask/agent without losing context but ask <-> agent is no problem. You can also change to your custom chat modes https://code.visualstudio.com/docs/copilot/chat/chat-modes without losing context. At least that's what I just did in VSCode Insiders.

Here are some nice copilot resources: https://github.com/github/awesome-copilot

Also, I am using tons of markdown documents for planning, results, research.... This makes it easy to get new agent sessions or yourself up to context.

Yes. I think it used to be separate tabs, but now chat/agent mode is just a toggle. After discussing a concept, you can just switch to agent mode and tell it to "implement the discussed plan."