Now that we have `/compact` we want to make the "new from summary"
option less prominent. You can still manually handoff to a new thread by
@mentioning the thread in a new one (`@thread threadname`)

Release Notes:

- agent: Remove "New from summary" menu item from agent panel dropdown.
You can still @mention the thread in a new one, or run /compact

Co-authored-by: gaauwe <gaauwe@users.noreply.github.com>