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/ping_monitor.py

@@ -67,6 +67,7 @@ class PingMonitorPlugin(MonitorPlugin):
         self.timeout = int(cfg.get("timeout", 5))
         # hosts: dict of {hostname: {warning: x, critical: y}} or list of hostnames
         raw_hosts = cfg.get("hosts", {})
+        self.hosts: Dict[str, Any]
         if isinstance(raw_hosts, list):
             self.hosts = {h: {} for h in raw_hosts}
         else: