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/config.py

@@ -2,15 +2,16 @@
 
 import logging
 import os
+from typing import Any, Dict
 
 logger = logging.getLogger(__name__)
 
 try:
     import yaml
 except Exception:
-    yaml = None
+    yaml = None  # type: ignore[assignment]
 
-CLIENT_DEFAULTS = {
+CLIENT_DEFAULTS: Dict[str, Any] = {
     # Network settings
     "hb_port": 50003,          # Port where hbd servers listen
     "interval": 10,             # Heartbeat interval in seconds