fix(gateway): replace wall-clock agent timeout with inactivity-based timeout (#5389)

The gateway previously used a hard wall-clock asyncio.wait_for timeout
that killed agents after a fixed duration regardless of activity. This
punished legitimate long-running tasks (subagent delegation, reasoning
models, multi-step research).

Now uses an inactivity-based polling loop that checks the agent's
built-in activity tracker (get_activity_summary) every 5 seconds. The
agent can run indefinitely as long as it's actively calling tools or
receiving API responses. Only fires when the agent has been completely
idle for the configured duration.

Changes:
- Replace asyncio.wait_for with asyncio.wait poll loop checking
  agent idle time via get_activity_summary()
- Add agent.gateway_timeout config.yaml key (default 1800s, 0=unlimited)
- Update stale session eviction to use agent idle time instead of
  pure wall-clock (prevents evicting active long-running tasks)
- Preserve all existing diagnostic logging and user-facing context

Inspired by PR #4864 (Mibayy) and issue #4815 (BongSuCHOI).
Reimplemented on current main using existing _touch_activity()
infrastructure rather than a parallel tracker.

hermes_cli/config.py

@@ -205,6 +205,11 @@ DEFAULT_CONFIG = {
     "toolsets": ["hermes-cli"],
     "agent": {
         "max_turns": 90,
+        # Inactivity timeout for gateway agent execution (seconds).
+        # The agent can run indefinitely as long as it's actively calling
+        # tools or receiving API responses.  Only fires when the agent has
+        # been completely idle for this duration.  0 = unlimited.
+        "gateway_timeout": 1800,
         # Tool-use enforcement: injects system prompt guidance that tells the
         # model to actually call tools instead of describing intended actions.
         # Values: "auto" (default — applies to gpt/codex models), true/false