Introduce gateway_timeout_warning (default 900s) as a pre-timeout alert
layer. When inactivity reaches the warning threshold, a single
notification is sent to the user offering to wait or reset. If
inactivity continues to the gateway_timeout (default 1800s), the full
timeout fires as before.
This gives users a chance to intervene before work is lost on slow
API providers without disabling the safety timeout entirely.
Config: agent.gateway_timeout_warning in config.yaml, or
HERMES_AGENT_TIMEOUT_WARNING env var (0 = disable warning).
