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/plugin.py

@@ -17,7 +17,7 @@ import logging
 import sys
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import Any, Dict, List, Optional, Type
+from typing import Any, Dict, List, Optional
 
 
 class Plugin(ABC):
@@ -256,7 +256,7 @@ class PluginRegistry:
         """Get all enabled plugins."""
         return [p for p in self._plugins.values() if p.enabled]
 
-    def get_by_type(self, plugin_type: Type[Plugin]) -> List[Plugin]:
+    def get_by_type(self, plugin_type: type) -> List[Plugin]:
         """Get all plugins of a specific type.
 
         Args: