fix(gateway): add staged inactivity warning before timeout escalation

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).

hermes_cli/config.py

@@ -230,6 +230,10 @@ DEFAULT_CONFIG = {
         # (force on/off for all models), or a list of model-name substrings
         # to match (e.g. ["gpt", "codex", "gemini", "qwen"]).
         "tool_use_enforcement": "auto",
+        # Staged inactivity warning: send a warning to the user at this
+        # threshold before escalating to a full timeout.  The warning fires
+        # once per run and does not interrupt the agent.  0 = disable warning.
+        "gateway_timeout_warning": 900,
     },
     
     "terminal": {