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/memory_monitor.py

@@ -10,7 +10,7 @@ from typing import Dict, Any, Optional
 try:
     import psutil
 except ImportError:
-    psutil = None
+    psutil = None  # type: ignore[assignment]
 
 from hbd.client.plugin import MonitorPlugin
 
@@ -116,7 +116,7 @@ class MemoryMonitorPlugin(MonitorPlugin):
 
     async def _collect_metrics(self) -> Dict[str, Any]:
         """Collect memory metrics from psutil."""
-        metrics = {}
+        metrics: Dict[str, Any] = {}
 
         # Virtual (physical) memory statistics
         vmem = psutil.virtual_memory()