bash: clarify timeout unit is ms to prevent exit-124 build kills

The timeout param is in milliseconds, but agents often pass small values
like 1000 thinking it means 1000s, causing builds to be SIGKILLed at 1s
and reported as 'exit 124 · 1.0s · No output captured'. Make the schema
description explicit and add a self-correcting hint to the timeout message.