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/filesystem_info.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 InfoPlugin
 
@@ -98,8 +98,8 @@ class FilesystemInfoPlugin(InfoPlugin):
 
     async def _collect_info(self) -> Dict[str, Any]:
         """Collect filesystem information from psutil."""
-        info = {}
-        filesystems = []
+        info: Dict[str, Any] = {}
+        filesystems: list = []
         filesystem_types = set()
 
         # Get mounted disk partitions
@@ -112,7 +112,7 @@ class FilesystemInfoPlugin(InfoPlugin):
             if partition.fstype in self.exclude_types:
                 continue
 
-            fs_info = {
+            fs_info: Dict[str, Any] = {
                 'device': partition.device,
                 'mountpoint': partition.mountpoint,
                 'fstype': partition.fstype,