fix: clear all mypy errors

Annotate untyped containers and Optional attributes, type the stdlib-shaped
metric dicts in the client plugins, and add types-PyYAML/types-psutil to the
dev extras plus a mypy override for nio/mattermostdriver (no stubs).

Behaviour fixes found on the way: enable_kernel_timestamps() now returns False
when SO_TIMESTAMP is unknown instead of raising TypeError, send_shutdown is
defined at module level so cleanup() cannot hit a NameError, and a failed
subprocess returncode falls back to UNKNOWN.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

hbd/client/plugins/cpu_monitor.py

@@ -28,7 +28,7 @@ class CPUMonitorPlugin(MonitorPlugin):
 
     def __init__(self, config: Optional[Dict[str, Any]] = None):
         super().__init__(config)
-        self.psutil = None
+        self.psutil: Any = None
         self.per_core = config.get("per_core", False) if config else False
         self.interval = config.get("interval", 300) if config else 300