/approve, /yes, /deny, /no (and /y, /n) are valid REPL-only slash
commands. Outside the REPL they were falling through to
format_unknown_direct_slash_command -> error_kind:unknown_slash_command.
Fix: intercept them in the SlashCommand::Unknown arm and emit
interactive_only: prefix so classify_error_kind returns the correct kind.
One new test: approve_deny_outside_repl_emits_interactive_only (covers
/approve, /yes, /deny, /no)
572 tests pass, 1 pre-existing worker_boot failure unrelated.