Port from cline/cline#10343: periodic gateway memory logging (#27102)

Emit a grep-friendly '[MEMORY] rss=...MB ...' line in agent.log /
gateway.log every N minutes (default 5) so slow leaks in the long-lived
gateway process show up as a time series. Based on
https://github.com/cline/cline/pull/10343
(src/standalone/memory-monitor.ts).

- gateway/memory_monitor.py: new module. Daemon thread, baseline on
  start, final snapshot on stop. Uses resource.getrusage() (stdlib)
  first, falls back to psutil, disables itself with one WARNING if
  neither is available.
- gateway/run.py: start monitor right after setup_logging() in
  start_gateway(); stop it in the shutdown block next to MCP teardown.
- hermes_cli/config.py: logging.memory_monitor { enabled, interval_seconds }
  defaults under the existing logging section.
- tests/gateway/test_memory_monitor.py: 10 unit tests covering format,
  baseline/shutdown snapshots, double-start noop, periodic timer,
  daemon thread invariant, and unavailable-RSS warn-and-skip path.

Adapted from TypeScript/Node to Python (threading.Event-based daemon
thread instead of setInterval/unref), added Python-specific gc + thread
counts to the log line (handier than ext/arrayBuffers for diagnosing
Python gateway leaks), and gated behind a config.yaml toggle so users
can silence the periodic line if they want.

No heap-snapshot-on-OOM equivalent — CPython doesn't have V8's
--heapsnapshot-near-heap-limit; tracemalloc would be the Python
equivalent but adds non-trivial overhead, so leaving that out.

hermes_cli/config.py

@@ -1477,6 +1477,15 @@ DEFAULT_CONFIG = {
         "level": "INFO",       # Minimum level for agent.log: DEBUG, INFO, WARNING
         "max_size_mb": 5,      # Max size per log file before rotation
         "backup_count": 3,     # Number of rotated backup files to keep
+        # Periodic process memory usage logging (gateway only). Emits a
+        # grep-friendly "[MEMORY] rss=...MB ..." line at the configured
+        # interval so slow leaks in the long-lived gateway are visible
+        # in agent.log / gateway.log as a time series. Ported from
+        # cline/cline#10343.
+        "memory_monitor": {
+            "enabled": True,         # Flip to false to silence the periodic line
+            "interval_seconds": 300, # Default: every 5 minutes
+        },
     },
 
     # Remotely-hosted model catalog manifest.  When enabled, the CLI fetches