sandboxes: warn users that merge is dumb

[brandonjackson]

Problem Statement

• Sandboxes merge is a blunt instrument
• The merge UI is not very clear about the risks of merging

Solution

• This will overwrite the selected workflows in brandon-test with the versions from foo, then delete foo. Any changes in brandon-test that conflict with foo will be lost.
• Replace beta warning with red "This action cannot be undone"

Ideas considered / not implemented:

• Replacing warning banner with "type name of sandbox" input. Decided against because this would probably get annoting. We want sandboxes to feel quick.
• Keeping CLI reference. Decided against because we need to focus on complete clarity about the risk.

[brandonjackson]

Before we settle on this we will need to align with the UI in #4002

[brandonjackson]

FYI @theroinaochieng moving to review b/c @doc-han has done this in his PR for merging specific workflows!