brev exec hangs and never executes multi-line / heredoc payloads

[myurasov-nv]

Problem

brev exec hangs indefinitely (no output, no timeout) and executes nothing on the instance when the payload is non-trivial. CLI v0.6.326 (macOS), massedcompute L40S.

Repro

Payload	Result
Multi-line -- bash -c 'cat > ~/x.sh << "EOF" ... EOF ...'	Hangs; ~/x.sh never created
@launch.sh containing heredoc + setsid nohup ... & disown	Hangs; first statement never runs; stray processes on the instance carry the script text in their command line
Linear @setup.sh (venv, pip, echo - no heredocs, no &)	Works

Follow-up single-line -- bash -c on the same instance: exit status 255.

Impact

Agents cannot stage-and-detach long jobs (training runs) via brev exec; only fallback is plain ssh to the instance endpoint.

Fix

Send @file content verbatim to the remote shell (e.g. pipe to bash -s) instead of re-quoting it; fail loudly on unrepresentable payloads instead of hanging.