This PR introduces a new `agent: chat with follow` action that
automatically enables "Follow Agent" when submitting a chat message with
`cmd-enter` or `ctrl-enter`. This is experimental. I'm not super
thrilled with the name, but the root action to submit a chat is called
`agent: chat`, so I'm following that wording. I'm also unsure if the
binding feels right or not.

Release Notes:

- Added an `agent: chat with follow` action via `cmd-enter` on macOS and
`ctrl-enter` on Linux