feat(cli): allow default log_file to be configured in workflow.yaml

[nskun]

Summary

--log-file can only be specified as a CLI argument Workflow authors cannot set a default in the workflow definition.
Users must remember to pass --log-file auto every time -- when forgotten, crashes leave no diagnostic log.

Problem

・When --log-file is not passed, crashes leave no diagnostic information.
・This is especially painful for long-running workflows(30-60 min) that crash near the end.
・default_model, max_tokens, temperature, timeout can already be configured in YAML, but log_file is the exception.

Proposal

Add a log_file field to the runtime: block.

workflow:
  runtime:
    log_file: auto

Accepts the same values as CLI --log-file:
・auto -- Generate a timestamped path in $TMPDIR/conductor/ (same as --log-file auto).
・A file path (e.g. ./logs/run.log) -- Write to that path directly.
・Omitted -- No file logging (current default, unchanged).

Precedence

CLI --log-file overrides the YAML value when both are specified.

・CLI --log-file specified -> Use CLI value
・CLI unset + YAML log_file set -> Use YAML value
・Neither set -> No logging (current behavior)