GCP lane: redirect workload output to log file — GCE metadata runner bufio.Scanner token-too-long kills workloads with progress bars

Goal

Make the GCP lane's startup script immune to the GCE metadata-runner bufio.Scanner: token too long kill by never streaming raw workload output through startup-script stdout.

Problem

Incident #491 attempt 2 (2026-06-11): the GCE metadata script runner reads startup-script stdout line-by-line with a bounded Go bufio.Scanner. vLLM's \r-carriage-return progress bars accumulated into one enormous newline-free line, overflowed the scanner (error while communicating with "startup-script" script: bufio.Scanner: token too long), the runner closed the pipe, and the workload died on SIGPIPE mid-free_gen. Because bash skips EXIT traps on a fatal SIGPIPE, eps/phase stayed workload and the VM zombied at RUNNING — invisible to the poll. Manual SSH recovery was required.

Fix (src/explore_persona_space/backends/gcp.py, render_startup_script)

1. Redirect the workload block's stdout/stderr to the handle's log_path ($WORKLOAD_ROOT/logs/issue-<N>.log) instead of streaming to the metadata runner; keep a small heartbeat (phase lines only) on stdout if useful.
2. Export TQDM_DISABLE=1 (and consider vLLM disable_tqdm) in the rendered env as defense in depth.
3. Consider trapping SIGPIPE (trap '' PIPE or explicit handler) so the rc!=0 EXIT trap (phase=failed + shutdown) still bounds billing if a kill class like this recurs.
4. Optional: have GcpBackend.poll read the log file's mtime over SSH for last_log_mtime_sec_ago so zombie states (phase stuck at workload, no live process) become detectable.

Acceptance criteria

• Rendered startup script no longer pipes workload output to the runner (unit test on render_startup_script output).
• A simulated giant-line workload (printf of >1MB with no newline) survives on a real or mocked launch path.
• Existing GCP-lane tests pass.

References

• Task #491 events: epm:failure + epm:failure-lesson markers, 2026-06-11.
• .claude/agent-memory/experimenter/feedback_gcp_metadata_runner_token_too_long.md.