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/disk_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
 
@@ -58,7 +58,7 @@ class DiskMonitorPlugin(MonitorPlugin):
             raise ImportError("psutil library is required for disk_monitor plugin")
 
         # Store previous I/O counters for delta calculation
-        self._prev_io = {}
+        self._prev_io: Dict[str, Any] = {}
 
     async def initialize(self):
         """Initialize the plugin (check psutil availability)."""
@@ -114,7 +114,7 @@ class DiskMonitorPlugin(MonitorPlugin):
 
     async def _collect_metrics(self) -> Dict[str, Any]:
         """Collect disk metrics from psutil."""
-        metrics = {}
+        metrics: Dict[str, Any] = {}
 
         # Collect partition usage
         partitions_data = {}