When a coding agent is missing, `unsloth start <agent>` offers to run the
vendor's own installer (curl | bash, irm | iex, or npm) after an interactive
confirm. Those installers execute with the user's privileges and there is no
signature or hash check on the fetched content, so a blind "yes" is a
supply-chain risk if the delivery path is compromised.
Keep the auto-install convenience but make consent informed: before the prompt,
name the exact remote source the installer fetches (or the command it runs for a
package installer) and state that nothing verifies a signature or hash. Behavior
is otherwise unchanged: non-interactive stdin still never executes anything, and
the confirm still defaults to no.
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