feat: add memtrace to all binaries, switch merlint commands to Eio
Add memtrace instrumentation to merlint, prune, crow, bottler, agent,
precommit, and uniboot. Refactor merlint's Command module from
Unix.open_process_in to Eio.Process.spawn for proper fiber scheduling.
Log external commands at INFO level (-v) and command output at DEBUG
level (-vv). Show dune build stderr in -vv mode instead of suppressing
with 2>/dev/null.