The `shell` step hardcoded a 300s subprocess timeout, so any command
that legitimately runs longer than five minutes (a full build, a linter
aggregator, an integration-test target) was killed with TimeoutExpired
and failed the whole run, with no YAML knob to raise the limit.
Add an optional `timeout` field (seconds) that defaults to 300 for
backward compatibility and is threaded through to `subprocess.run`. The
timeout failure message now reports the configured value instead of a
hardcoded 300. `validate` rejects a `timeout` that is not a positive
number (bool is rejected explicitly, since it is an int subclass but a
config error rather than a duration).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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